sbp2: fix another deadlock after disconnection

If there were commands enqueued but not completed before an SBP-2 unit
was unplugged (or an attempt to reconnect failed), knodemgrd or any
process which tried to remove the device would sleep uninterruptibly
in blk_execute_rq().  Therefore make sure that all commands are
completed when sbp2 retreats.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Jody McIntyre <scjody@modernduck.com>
(cherry picked from 61daa34c132c5d4ed8630e2c46e9bf2f0c7b3428 commit)