mm/z3fold.c: fix lock/unlock imbalance in z3fold_page_isolate

Fix lock/unlock imbalance by unlocking *zhdr* before return.

Addresses Coverity ID 1452811 ("Missing unlock")

Link: http://lkml.kernel.org/r/20190826030634.GA4379@embeddedor
Fixes: d776aaa9 ("mm/z3fold.c: fix race between migration and destruction")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Henry Burns <henrywolfeburns@gmail.com>
Cc: Vitaly Wool <vitalywool@gmail.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/z3fold.c

@@ -1406,6 +1406,7 @@ static bool z3fold_page_isolate(struct page *page, isolate_mode_t mode)
 				 * should freak out.
 				 */
 				WARN(1, "Z3fold is experiencing kref problems\n");
+				z3fold_page_unlock(zhdr);
 				return false;
 			}
 			z3fold_page_unlock(zhdr);