The ufs_lookup.c has a critical bug around the whiteout
process. UFS must check a whiteout name when it uses the
whiteout, but the current implementation does not check
the whileout name, so sometimes UFS writes over a wrong
whtieout. UFS *MUST* check the whiteout name to use a
corrent whiteout. This bug leads unionfs. panic.
This commit fixes this trouble.

Submitted by: Masanori Ozawa <ozawa@ongs.co.jp> (unionfs developer)
Reviewed by: tegge & rodrigc (mentor)
Approved by: rodrigc (mentor)
MFC after: 2 weeks