scsi_dh_alua: allocate RTPG buffer separately

The RTPG buffer will only evaluated within alua_rtpg(),
so we can allocate it locally there and avoid having to
put it into the global structure.

Reviewed-by: Ewan Milne <emilne@redhat.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>