block/nbd-client: simplify check in nbd_co_receive_reply

If we are woken up from while() loop in nbd_read_reply_entry
handles must be equal. If we are woken up from
nbd_recv_coroutines_wake_all s->quit must be true, so we do
not need checking handles equality.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20170920124507.18841-3-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
