No need to start a tracked request that will always fail.  The choice
to check read-only after bdrv_inc_in_flight() predates 1bc5f09f
(block: Use tracked request for truncate), but waiting for serializing
requests can make the effect more noticeable.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210609163034.997943-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>