fs/xfs/quota: Add missing mutex_unlock

Add a mutex_unlock missing on the error path.  The use of this lock
is balanced elsewhere in the file.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression E1;
@@

* mutex_lock(E1,...);
  <+... when != E1
  if (...) {
    ... when != E1
*   return ...;
  }
  ...+>
* mutex_unlock(E1,...);
// </smpl>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/quota/xfs_qm.c

@@ -249,8 +249,10 @@ xfs_qm_hold_quotafs_ref(
 
 	if (!xfs_Gqm) {
 		xfs_Gqm = xfs_Gqm_init();
-		if (!xfs_Gqm)
+		if (!xfs_Gqm) {
+			mutex_unlock(&xfs_Gqm_lock);
 			return ENOMEM;
+		}
 	}
 
 	/*