libbb: introduce and use bb_getsockname()

function                                             old     new   delta
bb_getsockname                                         -      18     +18
xrtnl_open                                            88      83      -5
do_iplink                                           1216    1209      -7
arping_main                                         1686    1668     -18
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 0/3 up/down: 18/-30)            Total: -12 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/bb_getsockname.c

@@ -0,0 +1,19 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Utility routines.
+ *
+ * Licensed under GPLv2, see file LICENSE in this source tree.
+ */
+//kbuild:lib-y += bb_getsockname.o
+
+#include "libbb.h"
+
+int FAST_FUNC bb_getsockname(int sockfd, void *addr, socklen_t addrlen)
+{
+	/* The usefullness of this function is that for getsockname(),
+	 * addrlen must go on stack (to _have_ an address to be passed),
+	 * but many callers do not need its modified value.
+	 * By using this shim, they can avoid unnecessary stack spillage.
+	 */
+	return getsockname(sockfd, (struct sockaddr *)addr, &addrlen);
+}