Enable witness(4) blessings.

witness has long had a facility to "bless" designated lock pairs. Lock
order reversals between a pair of blessed locks are not reported upon.
We have a number of long-standing false positive LOR reports; start
marking well-understood LORs as blessed.

This change hides reports about UFS vnode locks and the UFS dirhash
lock, and UFS vnode locks and buffer locks, since those are the two that
I observe most often. In the long term it would be preferable to be
able to limit blessings to a specific site where a lock is acquired,
and/or extend witness to understand why some lock order reversals are
valid (for example, if code paths with conflicting lock orders are
serialized by a third lock), but in the meantime the false positives
frequently confuse users and generate bug reports.

Reviewed by: cem, kib, mckusick
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21039