sanity check after every directory modification. This check can be
re-enabled at any time by setting the sysctl "vfs.ufs.dirhash_docheck"
to 1.
This group of sanity tests was there to ensure that any UFS_DIRHASH
bugs could be caught by a panic before a potentially corrupted
directory block would be written to disk. It has served its main
purpose now, so disable it in the interest of performance.
MFC after: 1 week
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