f2fs: fix to abort atomic write only during do_exist()

f2fs: fix to abort atomic write only during do_exist()

[ Upstream commit a768f479cd1fafbcfb5584987bb41a56a739f883 ]

Commit 0e0c433c381f ("f2fs: don't give partially written atomic data
from process crash") attempted to drop atomic write data after process
crash, however, f2fs_abort_atomic_write() may be called from noncrash
case, fix it by adding missed PF_EXITING check condition
f2fs_file_flush().

- application crashs
 - do_exit
  - exit_signals -- sets PF_EXITING
  - exit_files
   - put_files_struct
    - close_files
     - filp_close
      - flush (f2fs_file_flush)
       - check atomic_write_task && PF_EXITING
       - f2fs_abort_atomic_write

Fixes: 0e0c433c381f ("f2fs: don't give partially written atomic data from process crash")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>