Extend the sanity checks in ufs_lookup to ensure that each directory
entry fits within its DIRBLKSIZ block. The surrounding code is
extremely fragile with respect to corruption of the directory entry
'd_reclen' field; if directory corruption occurs, it can blindly
scan forward beyond the end of the filesystem block. Usually this
results in a 'fault on nofault entry' panic.

Directory corruption is now much more likely to be detected, resulting
in a 'ufs_dirbad' panic. If the filesystem is read-only, it will
simply print a warning message, and skip the corrupted block.

Reviewed by: mckusick