block/mirror: fix active mirror dead-lock in mirror_wait_on_conflicts

block/mirror: fix active mirror dead-lock in mirror_wait_on_conflicts

It's possible that requests start to wait each other in
mirror_wait_on_conflicts(). To avoid it let's use same technique as in
block/io.c in bdrv_wait_serialising_requests_locked() /
bdrv_find_conflicting_request(): don't wait on intersecting request if
it is already waiting for some other request.

For details of the dead-lock look at testIntersectingActiveIO()
test-case which we actually fixing now.

Fixes: d06107ade0ce74dc39739bac80de84b51ec18546
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210702211636.228981-4-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>