f2fs: fix to report error in f2fs_readdir

get_lock_data_page in f2fs_readdir can fail due to a lot of reasons (i.e.
no memory or IO error...), it's better to report this kind of error to
user rather than ignoring it.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>