Split SCSI header files

Move the constants that are used by both initiator and target
drivers into the new header file <scsi/scsi_proto.h>. This change
will allow to remove the initiator SCSI header include directives
from most SCSI target source files in a later patch.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>