scsi: target: tcmu: Implement tmr_notify callback

This patch implements the tmr_notify callback for tcmu.  When the callback
is called, tcmu checks the list of aborted commands it received as
parameter:

 - aborted commands in the qfull_queue are removed from the queue and
   target_complete_command is called

 - from the cmd_ids of aborted commands currently uncompleted in cmd ring
   it creates a list of aborted cmd_ids.

Finally a TMR notification is written to cmd ring containing TMR type and
cmd_id list. If there is no space in ring, the TMR notification is queued
on a TMR specific queue.

The TMR specific queue 'tmr_queue' can be seen as a extension of the cmd
ring. At the end of each iexecution of tcmu_complete_commands() we check
whether tmr_queue contains TMRs and try to move them onto the ring. If
tmr_queue is not empty after that, we don't call run_qfull_queue() because
commands must not overtake TMRs.

This way we guarantee that cmd_ids in TMR notification received by
userspace either match an active, not yet completed command or are no
longer valid due to userspace having complete some cmd_ids meanwhile.

New commands that were assigned to an aborted cmd_id will always appear on
the cmd ring _after_ the TMR.

Link: https://lore.kernel.org/r/20200726153510.13077-8-bstroesser@ts.fujitsu.com
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

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