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

sys/kern/kern_lockf.c

@@ -689,7 +689,7 @@ lf_advlockasync(struct vop_advlockasync_args *ap, struct lockf **statep,
 		break;
 	}
 
-#ifdef INVARIANTS
+#ifdef DIAGNOSTIC
 	/*
 	 * Check for some can't happen stuff. In this case, the active
 	 * lock list becoming disordered or containing mutually
@@ -917,7 +917,7 @@ lf_add_edge(struct lockf_entry *x, struct lockf_entry *y)
 	struct lockf_edge *e;
 	int error;
 
-#ifdef INVARIANTS
+#ifdef DIAGNOSTIC
 	LIST_FOREACH(e, &x->lf_outedges, le_outlink)
 		KASSERT(e->le_to != y, ("adding lock edge twice"));
 #endif