scsi: ibmvfc: Add FC payload retrieval routines for versioned vfcFrames

The FC iu and response payloads are located at different offsets depending
on the ibmvfc_cmd version. This is a result of the version 2 vfcFrame
definition adding an extra 64bytes of reserved space to the structure prior
to the payloads.

Add helper routines to determine the current vfcFrame version and return a
pointer to the proper iu or response structure within that ibmvfc_cmd.

Link: https://lore.kernel.org/r/20201118011104.296999-5-tyreld@linux.ibm.com


Acked-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>