mpt2sas: store scsi io tracker data in the scsi request

mpt2sas: store scsi io tracker data in the scsi request

Instead of storing the IO tracker structure in a separate list
that we need to pop/push to on every submit and complete (and
lock), store it in the pdu associated with a request. This is
possible on scsi-mq only, and further cuts the spinlock associated
time for higher IOPS IO workloads. At 100K IOPS, this effectively
cuts the locking time in half.

Signed-off-by: Jens Axboe <axboe@fb.com>