z3fold: fix spinlock unlocking in page reclaim

Commmit 5a27aa8 ("z3fold: add kref refcounting") introduced a bug
in z3fold_reclaim_page() with function exit that may leave pool->lock
spinlock held.  Here comes the trivial fix.

Fixes: 5a27aa8 ("z3fold: add kref refcounting")
Link: http://lkml.kernel.org/r/20170311222239.7b83d8e7ef1914e05497649f@gmail.com
Reported-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/z3fold.c

@@ -667,6 +667,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 			z3fold_page_unlock(zhdr);
 			spin_lock(&pool->lock);
 			if (kref_put(&zhdr->refcount, release_z3fold_page)) {
+				spin_unlock(&pool->lock);
 				atomic64_dec(&pool->pages_nr);
 				return 0;
 			}