Fix problem introduced in rev.1.406, which can cause already unlocked

mutex being unlocked again causing system panic.

sys/kern/vfs_subr.c

@@ -3148,6 +3148,7 @@ vfs_msync(struct mount *mp, int flags)
 			    curthread)) {
 				if (vp->v_vflag & VV_NOSYNC) {	/* unlinked */
 					vput(vp);
+					mtx_lock(&mntvnode_mtx);
 					continue;
 				}