libceph: avoid dropping con mutex before fault

The ceph_fault() function takes the con mutex, so we should avoid
dropping it before calling it.  This fixes a potential race with
another thread calling ceph_con_close(), or _open(), or similar (we
don't reverify con->state after retaking the lock).

Add annotation so that lockdep realizes we will drop the mutex before
returning.
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>

net/ceph/messenger.c

@@ -2336,7 +2336,6 @@ static void con_work(struct work_struct *work)
 	return;
 
 fault:
-	mutex_unlock(&con->mutex);
 	ceph_fault(con);     /* error/fault path */
 	goto done_unlocked;
 }
@@ -2347,9 +2346,8 @@ static void con_work(struct work_struct *work)
  * exponential backoff
  */
 static void ceph_fault(struct ceph_connection *con)
+	__releases(con->mutex)
 {
-	mutex_lock(&con->mutex);
-
 	pr_err("%s%lld %s %s\n", ENTITY_NAME(con->peer_name),
 	       ceph_pr_addr(&con->peer_addr.in_addr), con->error_msg);
 	dout("fault %p state %lu to peer %s\n",