Log GTP-U endpoints update

* make gtp_ntoa() public after renaming it to sgsn_gtp_ntoa() to avoid
  confusion with libgtp functions
* use it to log GTP-U endpoints address updates

Change-Id: I96d0f3a63cce338471cc39cc33fd44c39cd2aa73
Related: SYS#3610
diff --git a/src/gprs/gprs_gmm.c b/src/gprs/gprs_gmm.c
index d11ecce..147e001 100644
--- a/src/gprs/gprs_gmm.c
+++ b/src/gprs/gprs_gmm.c
@@ -120,6 +120,8 @@
 {
 	struct sgsn_pdp_ctx *pdp;
 	llist_for_each_entry(pdp, &mm_ctx->pdp_list, list) {
+		LOGMMCTXP(LOGL_INFO, mm_ctx, "Changing GTP-U endpoints %s -> %s\n",
+			  sgsn_gtp_ntoa(&pdp->lib->gsnlu), inet_ntoa(sgsn->cfg.gtp_listenaddr.sin_addr));
 		sgsn_pdp_upd_gtp_u(pdp,
 				   &sgsn->cfg.gtp_listenaddr.sin_addr,
 				   sizeof(sgsn->cfg.gtp_listenaddr.sin_addr));