mm/z3fold: fix possible null pointer dereferencing

alloc_slots could fail to allocate memory under heavy memory pressure.  So
we should check zhdr->slots against NULL to avoid future null pointer
dereferencing.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: fc5488651c ("z3fold: simplify freeing slots")
Signed-off-by: Miaohe Lin <[email protected]>
Reviewed-by: Vitaly Wool <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/z3fold.c

@@ -940,9 +940,19 @@ lookup:
 		}
 	}
 
-	if (zhdr && !zhdr->slots)
+	if (zhdr && !zhdr->slots) {
 		zhdr->slots = alloc_slots(pool, GFP_ATOMIC);
+		if (!zhdr->slots)
+			goto out_fail;
+	}
 	return zhdr;
+
+out_fail:
+	if (!kref_put(&zhdr->refcount, release_z3fold_page_locked)) {
+		add_to_unbuddied(pool, zhdr);
+		z3fold_page_unlock(zhdr);
+	}
+	return NULL;
 }
 
 /*