scsi: zfcp: use common code fcp_cmnd and fcp_resp with union in fsf_qtcb_bottom_io

This eases crash dump analysis by automatically dissecting these
protocol headers at least somewhat rather than getting a string
interpretation of large unstructured character array buffer fields.

Also, we can get rid of some unnecessary and error-prone type casts.

This change is possible since v2.6.33 commit 4318e08c


("[SCSI] zfcp: Update FCP protocol related code").

Signed-off-by: Steffen Maier <maier@linux.vnet.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.vnet.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>