ext4: fix null-ptr-deref in ext4_write_info

ext4: fix null-ptr-deref in ext4_write_info

I caught a null-ptr-deref bug as follows:
==================================================================
KASAN: null-ptr-deref in range [0x0000000000000068-0x000000000000006f]
CPU: 1 PID: 1589 Comm: umount Not tainted 5.10.0-02219-dirty #339
RIP: 0010:ext4_write_info+0x53/0x1b0
[...]
Call Trace:
 dquot_writeback_dquots+0x341/0x9a0
 ext4_sync_fs+0x19e/0x800
 __sync_filesystem+0x83/0x100
 sync_filesystem+0x89/0xf0
 generic_shutdown_super+0x79/0x3e0
 kill_block_super+0xa1/0x110
 deactivate_locked_super+0xac/0x130
 deactivate_super+0xb6/0xd0
 cleanup_mnt+0x289/0x400
 __cleanup_mnt+0x16/0x20
 task_work_run+0x11c/0x1c0
 exit_to_user_mode_prepare+0x203/0x210
 syscall_exit_to_user_mode+0x5b/0x3a0
 do_syscall_64+0x59/0x70
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
 ==================================================================

Above issue may happen as follows:
-------------------------------------
exit_to_user_mode_prepare
 task_work_run
  __cleanup_mnt
   cleanup_mnt
    deactivate_super
     deactivate_locked_super
      kill_block_super
       generic_shutdown_super
        shrink_dcache_for_umount
         dentry = sb->s_root
         sb->s_root = NULL              <--- Here set NULL
        sync_filesystem
         __sync_filesystem
          sb->s_op->sync_fs > ext4_sync_fs
           dquot_writeback_dquots
            sb->dq_op->write_info > ext4_write_info
             ext4_journal_start(d_inode(sb->s_root), EXT4_HT_QUOTA, 2)
              d_inode(sb->s_root)
               s_root->d_inode          <--- Null pointer dereference

To solve this problem, we use ext4_journal_start_sb directly
to avoid s_root being used.

Cc: stable@kernel.org
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220805123947.565152-1-libaokun1@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>