mm, swap: improve readability via make spin_lock/unlock balanced

This is just a cleanup patch, no functionality change.

In cluster_list_add_tail(), spin_lock_nested() is used to lock the
cluster, while unlock_cluster() is used to unlock the cluster.  To
improve the code readability, use spin_unlock() directly to unlock the
cluster.

Link: http://lkml.kernel.org/r/20170317064635.12792-2-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Tim Chen <tim.c.chen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -335,7 +335,7 @@ static void cluster_list_add_tail(struct swap_cluster_list *list,
 		ci_tail = ci + tail;
 		spin_lock_nested(&ci_tail->lock, SINGLE_DEPTH_NESTING);
 		cluster_set_next(ci_tail, idx);
-		unlock_cluster(ci_tail);
+		spin_unlock(&ci_tail->lock);
 		cluster_set_next_flag(&list->tail, idx, 0);
 	}
 }