Verify that '..' still exists with the same inode number after
VFS_VGET() has returned in ufs_lookup(). If the '..' lookup started
immediately before the parent directory was removed, we might return
either cleared or unrelated inode otherwise.

Ufs_lookup() is split into new function ufs_lookup_() that either does
lookup, or verifies that directory entry exists and references supplied
inode number.

Reviewed by: tegge
Tested by: pho,
Andreas Tobler <andreast-list fgznet ch> (previous version)
MFC after: 1 month