udmabuf: add back sanity check

Check vm_fault->pgoff before using it.  When we removed the warning, we
also removed the check.

Fixes: 7b26e4e2 ("udmabuf: drop WARN_ON() check.")
Reported-by:  <zdi-disclosures@trendmicro.com>
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>