filldir[64]: remove WARN_ON_ONCE() for bad directory entries

This was always meant to be a temporary thing, just for testing and to
see if it actually ever triggered.

The only thing that reported it was syzbot doing disk image fuzzing, and
then that warning is expected.  So let's just remove it before -rc4,
because the extra sanity testing should probably go to -stable, but we
don't want the warning to do so.

Reported-by:  <syzbot+3031f712c7ad5dd4d926@syzkaller.appspotmail.com>
Fixes: 8a23eb80 ("Make filldir[64]() verify the directory entry filename is valid")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>