erofs: fix 'backmost' member of z_erofs_decompress_frontend

erofs: fix 'backmost' member of z_erofs_decompress_frontend

Initialize 'backmost' to true in DECOMPRESS_FRONTEND_INIT.

Fixes: 5c6dcc57e2e5 ("erofs: get rid of `struct z_erofs_collector'")
Signed-off-by: Weizhao Ouyang <o451686892@gmail.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220530075114.918874-1-o451686892@gmail.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>