scsi_transport_sas: add is_sas_attached() function

Adds a function designed to be callable any time (regardless of
whether the transport attributes are configured or not) which returns
true if the device is attached over a SAS transport.  The design of
this function is that transport specific functions can be embedded
within a

if (is_sas_attached(sdev)) {
	...
}

which would be compiled out (and thus eliminate the symbols) if SAS is
not configured.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

include/scsi/scsi_transport_sas.h

@@ -10,6 +10,15 @@ struct scsi_transport_template;
 struct sas_rphy;
 struct request;
 
+#if !IS_ENABLED(CONFIG_SCSI_SAS_ATTRS)
+static inline int is_sas_attached(struct scsi_device *sdev)
+{
+	return 0;
+}
+#else
+extern int is_sas_attached(struct scsi_device *sdev);
+#endif
+
 static inline int sas_protocol_ata(enum sas_protocol proto)
 {
 	return ((proto & SAS_PROTOCOL_SATA) ||