xen-blkback: fix leak on grant map error path

Fix leaking a page when a grant mapping has failed.

CC: stable@vger.kernel.org
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reported-and-Tested-by: Tao Chen <boby.chen@huawei.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/blkback.c

@@ -763,6 +763,7 @@ static int xen_blkbk_map(struct xen_blkif *blkif,
 			BUG_ON(new_map_idx >= segs_to_map);
 			if (unlikely(map[new_map_idx].status != 0)) {
 				pr_debug(DRV_PFX "invalid buffer -- could not remap it\n");
+				put_free_pages(blkif, &pages[seg_idx]->page, 1);
 				pages[seg_idx]->handle = BLKBACK_INVALID_HANDLE;
 				ret |= 1;
 				goto next;