scsi: ibmvfc: Add cancel mad initialization helper

Add a helper routine for initializing a Cancel MAD. This will be useful for
a channelized client that needs to send Cancel commands down every channel
commands were sent for a particular LUN.

Link: https://lore.kernel.org/r/20210114203148.246656-18-tyreld@linux.ibm.com


Reviewed-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>