gb: optionally allow nsip packets only from a specific host

When listening for nsip connections is enabled, then every remote
host may send packets. This is useful for an SGSN that serves
multiple PCUs, but contraproductive for a PCU that awaits packets
from a single SGSN.

Add struct members remote_ip, and remote_port to struct gprs_ns_inst,
when set, then the listening end uses connect() to ensure that only
the expected host may send packets.

Related: OS#2401
Change-Id: Ifeb201d9006eec275a46708007ff342cdfc14e45
diff --git a/include/osmocom/gprs/gprs_ns.h b/include/osmocom/gprs/gprs_ns.h
index 5aee755..d4aef82 100644
--- a/include/osmocom/gprs/gprs_ns.h
+++ b/include/osmocom/gprs/gprs_ns.h
@@ -88,6 +88,8 @@
 		struct osmo_fd fd;
 		uint32_t local_ip;
 		uint16_t local_port;
+		uint32_t remote_ip;
+		uint16_t remote_port;
 		int dscp;
 	} nsip;
 	/*! NS-over-FR-over-GRE-over-IP specific bits */