f2fs: avoid clear valid page

In f2fs_delete_entry, if last dirent is remove from the dentry page,
we will try to punch that page since it has no valid date in it.

But truncate_hole which is used for punching could fail because of
no memory or IO error, if that happened, we'd better skip clearing
this valid dentry page.

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