libata: move sas ata tag allocation to libata-scsi.c

Basically move the sas ata tag allocation to libata-scsi.c to make it clear
these staffs are just for sas.

Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/ata/libata.h b/drivers/ata/libata.h
index 4040513..8c491cd 100644
--- a/drivers/ata/libata.h
+++ b/drivers/ata/libata.h
@@ -145,6 +145,8 @@
 extern int ata_bus_probe(struct ata_port *ap);
 extern int ata_scsi_user_scan(struct Scsi_Host *shost, unsigned int channel,
 			      unsigned int id, u64 lun);
+int ata_sas_allocate_tag(struct ata_port *ap);
+void ata_sas_free_tag(unsigned int tag, struct ata_port *ap);
 
 
 /* libata-eh.c */