fusion: Remove use of DEF_SCSI_QCMD

Removing the host_lock from the I/O submission path gives a huge
scalability improvement.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Reviewed-by: Nicholas Bellinger <nab@linux-iscsi.org>
Reviewed-by: Praveen Krishnamoorthy <Praveen.krishnamoorthy@lsi.com>
Acked-by: Sreekanth Reddy <Sreekanth.reddy@lsi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>