cache: get rid of the global lock

Add a table of vnode locks and use them along with bucketlocks to provide
concurrent modification support. The approach taken is to preserve the
current behaviour of the namecache and just lock all relevant parts before
any changes are made.

Lookups still require the relevant bucket to be locked.

Discussed with:		kib
Tested by:	pho

sys/kern/subr_witness.c

@@ -625,7 +625,7 @@ static struct witness_order_list_entry order_lists[] = {
 	/*
 	 * VFS namecache
 	 */
-	{ "ncglobal", &lock_class_rw },
+	{ "ncvn", &lock_class_mtx_sleep },
 	{ "ncbuc", &lock_class_rw },
 	{ "vnode interlock", &lock_class_mtx_sleep },
 	{ "ncneg", &lock_class_mtx_sleep },