Add abort support for aens. Commonized the op abort to apply to aen or
real ios (caused some reorg/routine movement). Abort path sets termination
flag in prep for next patch that will be watching i/o abort completion
before proceeding with controller teardown.

Now that we're aborting aens, the "exit" code that simply cleared out
their context no longer applies.

Also clarified how we detect an AEN vs a normal io - by a flag, not
by whether a rq exists or the a rqno is out of range.

Note: saw some interesting cases where if the queues are stopped and
we're waiting for the aborts, the core layer can call the complete_rq
callback for the io. So the io completion synchronizes link side completion
with possible blk layer completion under error.
Signed-off-by: NJames Smart <james.smart@broadcom.com>
Reviewed-by: NSagi Grimberg <sagi@grimberg.me>
Signed-off-by: NChristoph Hellwig <hch@lst.de>