jffs2: check the validity of dstlen in jffs2_zlib_compress()

jffs2: check the validity of dstlen in jffs2_zlib_compress()

KASAN reports a BUG when download file in jffs2 filesystem.It is
because when dstlen == 1, cpage_out will write array out of bounds.
Actually, data will not be compressed in jffs2_zlib_compress() if
data's length less than 4.

[  393.799778] BUG: KASAN: slab-out-of-bounds in jffs2_rtime_compress+0x214/0x2f0 at addr ffff800062e3b281
[  393.809166] Write of size 1 by task tftp/2918
[  393.813526] CPU: 3 PID: 2918 Comm: tftp Tainted: G    B           4.9.115-rt93-EMBSYS-CGEL-6.1.R6-dirty #1
[  393.823173] Hardware name: LS1043A RDB Board (DT)
[  393.827870] Call trace:
[  393.830322] [<ffff20000808c700>] dump_backtrace+0x0/0x2f0
[  393.835721] [<ffff20000808ca04>] show_stack+0x14/0x20
[  393.840774] [<ffff2000086ef700>] dump_stack+0x90/0xb0
[  393.845829] [<ffff20000827b19c>] kasan_object_err+0x24/0x80
[  393.851402] [<ffff20000827b404>] kasan_report_error+0x1b4/0x4d8
[  393.857323] [<ffff20000827bae8>] kasan_report+0x38/0x40
[  393.862548] [<ffff200008279d44>] __asan_store1+0x4c/0x58
[  393.867859] [<ffff2000084ce2ec>] jffs2_rtime_compress+0x214/0x2f0
[  393.873955] [<ffff2000084bb3b0>] jffs2_selected_compress+0x178/0x2a0
[  393.880308] [<ffff2000084bb530>] jffs2_compress+0x58/0x478
[  393.885796] [<ffff2000084c5b34>] jffs2_write_inode_range+0x13c/0x450
[  393.892150] [<ffff2000084be0b8>] jffs2_write_end+0x2a8/0x4a0
[  393.897811] [<ffff2000081f3008>] generic_perform_write+0x1c0/0x280
[  393.903990] [<ffff2000081f5074>] __generic_file_write_iter+0x1c4/0x228
[  393.910517] [<ffff2000081f5210>] generic_file_write_iter+0x138/0x288
[  393.916870] [<ffff20000829ec1c>] __vfs_write+0x1b4/0x238
[  393.922181] [<ffff20000829ff00>] vfs_write+0xd0/0x238
[  393.927232] [<ffff2000082a1ba8>] SyS_write+0xa0/0x110
[  393.932283] [<ffff20000808429c>] __sys_trace_return+0x0/0x4
[  393.937851] Object at ffff800062e3b280, in cache kmalloc-64 size: 64
[  393.944197] Allocated:
[  393.946552] PID = 2918
[  393.948913]  save_stack_trace_tsk+0x0/0x220
[  393.953096]  save_stack_trace+0x18/0x20
[  393.956932]  kasan_kmalloc+0xd8/0x188
[  393.960594]  __kmalloc+0x144/0x238
[  393.963994]  jffs2_selected_compress+0x48/0x2a0
[  393.968524]  jffs2_compress+0x58/0x478
[  393.972273]  jffs2_write_inode_range+0x13c/0x450
[  393.976889]  jffs2_write_end+0x2a8/0x4a0
[  393.980810]  generic_perform_write+0x1c0/0x280
[  393.985251]  __generic_file_write_iter+0x1c4/0x228
[  393.990040]  generic_file_write_iter+0x138/0x288
[  393.994655]  __vfs_write+0x1b4/0x238
[  393.998228]  vfs_write+0xd0/0x238
[  394.001543]  SyS_write+0xa0/0x110
[  394.004856]  __sys_trace_return+0x0/0x4
[  394.008684] Freed:
[  394.010691] PID = 2918
[  394.013051]  save_stack_trace_tsk+0x0/0x220
[  394.017233]  save_stack_trace+0x18/0x20
[  394.021069]  kasan_slab_free+0x88/0x188
[  394.024902]  kfree+0x6c/0x1d8
[  394.027868]  jffs2_sum_write_sumnode+0x2c4/0x880
[  394.032486]  jffs2_do_reserve_space+0x198/0x598
[  394.037016]  jffs2_reserve_space+0x3f8/0x4d8
[  394.041286]  jffs2_write_inode_range+0xf0/0x450
[  394.045816]  jffs2_write_end+0x2a8/0x4a0
[  394.049737]  generic_perform_write+0x1c0/0x280
[  394.054179]  __generic_file_write_iter+0x1c4/0x228
[  394.058968]  generic_file_write_iter+0x138/0x288
[  394.063583]  __vfs_write+0x1b4/0x238
[  394.067157]  vfs_write+0xd0/0x238
[  394.070470]  SyS_write+0xa0/0x110
[  394.073783]  __sys_trace_return+0x0/0x4
[  394.077612] Memory state around the buggy address:
[  394.082404]  ffff800062e3b180: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
[  394.089623]  ffff800062e3b200: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
[  394.096842] >ffff800062e3b280: 01 fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[  394.104056]                    ^
[  394.107283]  ffff800062e3b300: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[  394.114502]  ffff800062e3b380: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[  394.121718] ==================================================================

Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: Richard Weinberger <richard@nod.at>