locks: fix NULL-deref in generic_delete_lease

commit 0efaa7e82f02fe69c05ad28e905f31fc86e6f08e
  locks: generic_delete_lease doesn't need a file_lock at all

moves the call to fl->fl_lmops->lm_change() to a place in the
code where fl might be a non-lease lock.
When that happens, fl_lmops is NULL and an Oops ensures.

So add an extra test to restore correct functioning.

Reported-by: Linda Walsh <suse@tlinx.org>
Link: https://bugzilla.suse.com/show_bug.cgi?id=912569
Cc: stable@vger.kernel.org (v3.18)
Fixes: 0efaa7e82f02fe69c05ad28e905f31fc86e6f08e
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>

fs/locks.c

@@ -1702,7 +1702,7 @@ static int generic_delete_lease(struct file *filp)
 			break;
 	}
 	trace_generic_delete_lease(inode, fl);
-	if (fl)
+	if (fl && IS_LEASE(fl))
 		error = fl->fl_lmops->lm_change(before, F_UNLCK, &dispose);
 	spin_unlock(&inode->i_lock);
 	locks_dispose_list(&dispose);