f2fs: avoid false shutdown fserror reports

f2fs: avoid false shutdown fserror reports

F2FS records image errors and checkpoint-stop reasons through the same
s_error_work worker.  The ordinary f2fs_handle_error() path only updates
s_errors, but the worker still calls fserror_report_shutdown()
unconditionally after committing the superblock.

As a result, a metadata corruption report can be followed by a synthetic
FAN_FS_ERROR event with ESHUTDOWN and an invalid superblock file handle,
even though no stop reason was recorded.

Track whether save_stop_reason() actually changed the stop_reason array
and only report the shutdown fserror for that case.  Pure s_errors updates
still commit the superblock, but no longer generate a false shutdown event.

Fixes: 50faed607d32 ("f2fs: support to report fserror")
Cc: stable@kernel.org
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>