[SCSI] zfcp: allocate gid_pn_data objects from gid_pn_cache

allocate gid_pn_data objects from gid_pn_cache.

Allocate gid_pn_data objects from the corresponding cache which ensures
proper alignment.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>