greybus: svc: pwrmon: validate svc protocol op status when getting rail names

AP should check for Greybus SVC Protocol Operation Status to determine if the
operation was successfully completed by the SVC

Testing Done:
- Successfully getting the rail names in the pwrmon_dummy sandbox branch

Signed-off-by: David Lin <[e-mail]>
Reviewed-by: Viresh Kumar <[e-mail]>
Reviewed-by: Bartosz Golaszewski <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/greybus/greybus_protocols.h

···
 #define GB_SVC_PWRMON_RAIL_NAME_BUFSIZE		32
 
 struct gb_svc_pwrmon_rail_names_get_response {
+	__u8	status;
 	__u8	name[0][GB_SVC_PWRMON_RAIL_NAME_BUFSIZE];
 } __packed;
 

drivers/staging/greybus/svc.c

···
 		return ret;
 	}
 
+	if (response->status != GB_SVC_OP_SUCCESS) {
+		dev_err(&svc->dev,
+			"SVC error while getting rail names: %u\n",
+			response->status);
+		return -EREMOTEIO;
+	}
+
 	return 0;
 }
 
···
 	if (!rail_count || rail_count > GB_SVC_PWRMON_MAX_RAIL_COUNT)
 		goto err_pwrmon_debugfs;
 
-	bufsize = GB_SVC_PWRMON_RAIL_NAME_BUFSIZE * rail_count;
+	bufsize = sizeof(*rail_names) +
+		GB_SVC_PWRMON_RAIL_NAME_BUFSIZE * rail_count;
 
 	rail_names = kzalloc(bufsize, GFP_KERNEL);
 	if (!rail_names)