vfs: consult freevnodes in vnlru_kick_cond

If the count is high enough there is no point trying to produce more.
Not going there reduces traffic on the vnode_list mtx.

This further shaves total real time in a test mentioned in:
74be676d87 ("vfs: drop one vnode list lock trip during vnlru free
recycle") -- 20 instances of find each creating 1 million vnodes, while
total limit is set to 400k.

Time goes down from ~41 to ~35 seconds.

Sponsored by:	Rubicon Communications, LLC ("Netgate")

sys/kern/vfs_subr.c

@@ -1584,6 +1584,9 @@ static void
 vnlru_kick_cond(void)
 {
 
+	if (vnlru_read_freevnodes() > wantfreevnodes)
+		return;
+
 	if (vnlruproc_sig)
 		return;
 	mtx_lock(&vnode_list_mtx);
@@ -1849,9 +1852,8 @@ vn_alloc_hard(struct mount *mp)
 	}
 alloc:
 	mtx_assert(&vnode_list_mtx, MA_NOTOWNED);
-	rnumvnodes = atomic_fetchadd_long(&numvnodes, 1) + 1;
-	if (vnlru_under(rnumvnodes, vlowat))
-		vnlru_kick_cond();
+	atomic_add_long(&numvnodes, 1);
+	vnlru_kick_cond();
 	return (uma_zalloc_smr(vnode_zone, M_WAITOK));
 }