SIOCSIFNAME: Do nothing if we're not actually changing

Instead of throwing EEXIST, just succeed if the name isn't actually
changing. We don't need to trigger departure or any of that because there's
no change from consumers' perspective.

PR:		240539
Reviewed by:	brooks
MFC after:	5 days
Differential Revision:	https://reviews.freebsd.org/D21618

sys/net/if.c

@@ -2716,6 +2716,8 @@ ifhwioctl(u_long cmd, struct ifnet *ifp, caddr_t data, struct thread *td)
 			if (strlen(new_name) == IFNAMSIZ-1)
 				return (EINVAL);
 		}
+		if (strcmp(new_name, ifp->if_xname) == 0)
+			break;
 		if (ifunit(new_name) != NULL)
 			return (EEXIST);