libata: make ata_print_id atomic

This variable is incremented from multiple contexts (module_init via
libata-lldds and the libsas discovery thread).  Make it atomic to head
off any chance of libsas and libata creating duplicate ids.

Acked-by: Jacek Danecki <jacek.danecki@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index 1ee00c8..93dabdc 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -3843,7 +3843,7 @@
 	int rc = ap->ops->port_start(ap);
 
 	if (!rc) {
-		ap->print_id = ata_print_id++;
+		ap->print_id = atomic_inc_return(&ata_print_id);
 		__ata_port_probe(ap);
 	}
 
@@ -3867,7 +3867,7 @@
 	int rc = ap->ops->port_start(ap);
 
 	if (!rc) {
-		ap->print_id = ata_print_id++;
+		ap->print_id = atomic_inc_return(&ata_print_id);
 		rc = ata_port_probe(ap);
 	}