scsi: fnic: replace gross legacy tag hack with blk-mq hack

Would be nice to fix up the SCSI midlayer instead, but this will do for
now.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Satish Kharat <satishkh@cisco.com>
Cc: linux-scsi@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>