put very expensive sanity checks of advisory locks under DIAGNOSTIC

The checks have quadratic complexity over a number of advisory locks
active for a file and that could be a lot. What's the worse is that the
checks are done while holding ls_lock. That could lead to a long a very
long backlog and performance degradation even if all requested locks are
compatible (e.g. all shared locks).

The checks used to be under INVARIANTS.

Discussed with: kib
MFC after: 2 weeks
Sponsored by: Panzura