pnfs/blocklayout: Add module alias for LAYOUT4_SCSI

The blocklayout module contains the client support for both block and SCSI
layouts.  Add a module alias for the SCSI layout type so that the module
will be loaded for SCSI layouts.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>