[SCSI] dh_rdac: Use WWID from C8 page instead of Subsystem id from C4 page to identify storage

rdac hardware handler uses "Subsystem Identifier" from C4 inquiry page
to uniquely identify a storage. The problem with that is that if any
any of the bytes are non-ascii, subsys_id will all be spaces (hex
0x20). This creates lot of problems especially when there are multiple
rdac storages are connected to the server.

Use "Storage Array Unique Identifier" from C8 inquiry page, which is the
world wide unique identifier for the storage array, to uniquely identify
the storage.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>