fs/ocfs2/dlm: Add missing spin_unlock

Add a spin_unlock missing on the error path.  Unlock as in the other code
that leads to the leave label.

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

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

* spin_lock(E1,...);
  <+... when != E1
  if (...) {
    ... when != E1
*   return ...;
  }
  ...+>
* spin_unlock(E1,...);
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

fs/ocfs2/dlm/dlmdomain.c

@@ -1709,6 +1709,7 @@ retry:
 		}
 
 		if (dlm_protocol_compare(&dlm->fs_locking_proto, fs_proto)) {
+			spin_unlock(&dlm_domain_lock);
 			mlog(ML_ERROR,
 			     "Requested locking protocol version is not "
 			     "compatible with already registered domain "