f2fs: fix to avoid UAF in f2fs_write_end_io()

f2fs: fix to avoid UAF in f2fs_write_end_io()

As syzbot reported an use-after-free issue in f2fs_write_end_io().

It is caused by below race condition:

loop device				umount
- worker_thread
 - loop_process_work
  - do_req_filebacked
   - lo_rw_aio
    - lo_rw_aio_complete
     - blk_mq_end_request
      - blk_update_request
       - f2fs_write_end_io
        - dec_page_count
        - folio_end_writeback
					- kill_f2fs_super
					 - kill_block_super
					  - f2fs_put_super
					 : free(sbi)
       : get_pages(, F2FS_WB_CP_DATA)
         accessed sbi which is freed

In kill_f2fs_super(), we will drop all page caches of f2fs inodes before
call free(sbi), it guarantee that all folios should end its writeback, so
it should be safe to access sbi before last folio_end_writeback().

Let's relocate ckpt thread wakeup flow before folio_end_writeback() to
resolve this issue.

Cc: stable@kernel.org
Fixes: e234088758fc ("f2fs: avoid wait if IO end up when do_checkpoint for better performance")
Reported-by: syzbot+b4444e3c972a7a124187@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b4444e3c972a7a124187
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>