nbd: safeguard against waking up invalid coroutine

nbd: safeguard against waking up invalid coroutine

The .reply_possible field of s->requests is never set to false.  This is
not a problem as it is only a safeguard to detect protocol errors,
but it's sloppy.  In fact, the field is actually not necessary at all,
because .coroutine is set to NULL in NBD_FOREACH_REPLY_CHUNK after
receiving the last chunk.  Thus, replace .reply_possible with .coroutine
and move the check before deciding the fate of this request.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414175756.671165-2-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Signed-off-by: Eric Blake <eblake@redhat.com>