f2fs: support to report fserror

f2fs: support to report fserror

This patch supports to report fserror, it provides another way to let
userspace to monitor filesystem level error. In addition, it exports
/sys/fs/f2fs/features/fserror once f2fs kernel module start to support
the new feature, then generic/791 of fstests can notice the feature,
and verify validation of fserror report.

Cc: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>