block: fix shadowed variable in bdrv_co_pdiscard

We've had a shadowed 'ret' variable, which risks returning the wrong
value, introduced in commit b9c6494.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170710150559.30163-1-den@openvz.org
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/io.c

@@ -2335,7 +2335,6 @@ int coroutine_fn bdrv_co_pdiscard(BlockDriverState *bs, int64_t offset,
     assert(max_pdiscard >= bs->bl.request_alignment);
 
     while (bytes > 0) {
-        int ret;
         int num = bytes;
 
         if (head) {