osmo-sgsn: ping GGSN periodically and check for restart counter

Before this commit, echo req/rsp logic was implemented in libgtp but
never used in osmo-sgsn.

This commit adds a timer which periodically sends a GTP ECHO Request to
every GGSN if there's at least one pdpd context associated with it. This
way by checking the restart counter in the ECHO Reply it can be known if
the GGSN was restarted. In this case, logic already present in osmo-sgsn
will terminate all pdp contexts associated with that GGSN.

Change-Id: I9d714726785407859f26bbef052cd0efc28e8dae
diff --git a/include/osmocom/sgsn/sgsn.h b/include/osmocom/sgsn/sgsn.h
index 829a8fc..dae9d12 100644
--- a/include/osmocom/sgsn/sgsn.h
+++ b/include/osmocom/sgsn/sgsn.h
@@ -171,6 +171,7 @@
 					 struct tlv_parsed *tp);
 int sgsn_delete_pdp_ctx(struct sgsn_pdp_ctx *pctx);
 void sgsn_pdp_upd_gtp_u(struct sgsn_pdp_ctx *pdp, void *addr, size_t alen);
+void sgsn_ggsn_echo_req(struct sgsn_ggsn_ctx *ggc);
 
 /* gprs_sndcp.c */