nbd: Fix 32-bit compilation on BLOCK_STATUS

iotests 123 and 209 fail on 32-bit platforms.  The culprit:
sizeof(extent) is wrong; we want sizeof(*extent).  But since
the struct is 8 bytes, it happened to work on 64-bit platforms
where the pointer is also 8 bytes (nasty).

Fixes: 78a33ab5Reported-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20180327210517.1804242-1-eblake@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

block/nbd-client.c

@@ -239,7 +239,7 @@ static int nbd_parse_blockstatus_payload(NBDClientSession *client,
 {
     uint32_t context_id;
 
-    if (chunk->length != sizeof(context_id) + sizeof(extent)) {
+    if (chunk->length != sizeof(context_id) + sizeof(*extent)) {
         error_setg(errp, "Protocol error: invalid payload for "
                          "NBD_REPLY_TYPE_BLOCK_STATUS");
         return -EINVAL;