scsi_dh_alua: separate out alua_stpg()

Separate out SET TARGET PORT GROUP functionality into a separate
function alua_stpg().

Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>