Revert r334708

This is the wrong place to put the barrier.
Requested by:	kib,mjg

sys/kern/vfs_subr.c

@@ -2807,9 +2807,6 @@ _vhold(struct vnode *vp, bool locked)
 	CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
 	if (!locked) {
 		if (refcount_acquire_if_not_zero(&vp->v_holdcnt)) {
-#if !defined(__amd64__) && !defined(__i386__)
-			mb();
-#endif
 			VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
 			    ("_vhold: vnode with holdcnt is free"));
 			return;