scsi: core: switch to scsi-mq by default

It has been more than one year since we tried to change the default from
legacy to multi queue in SCSI with commit c279bd9e406 ("scsi: default to
scsi-mq"). But due to issues with suspend/resume and performance problems
it had been reverted again with commit cbe7dfa2

 ("Revert "scsi: default
to scsi-mq"").

In the meantime there have been a substantial amount of performance
improvements and suspend/resume got fixed as well, thus we can re-enable
scsi-mq without a significant performance penalty.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Acked-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>