socketutils: introduce get_family_by_proto

Add a helper function that returns address family based on the internal
notion of the socket protocol using protocols table.

* defs.h (get_family_by_proto): New declaration.
* socketutils.c (get_family_by_proto): New function.

defs.h

@@ -396,6 +396,7 @@ enum sock_proto {
 	SOCK_PROTO_NETLINK,
 };
 extern enum sock_proto get_proto_by_name(const char *);
+extern int get_family_by_proto(enum sock_proto proto);
 
 enum iov_decode {
 	IOV_DECODE_ADDR,

socketutils.c

@@ -507,6 +507,15 @@ get_proto_by_name(const char *const name)
 	return SOCK_PROTO_UNKNOWN;
 }
 
+int
+get_family_by_proto(enum sock_proto proto)
+{
+	if ((size_t) proto < ARRAY_SIZE(protocols))
+		return protocols[proto].family;
+
+	return AF_UNSPEC;
+}
+
 static const char *
 get_sockaddr_by_inode_uncached(struct tcb *tcp, const unsigned long inode,
 			       const enum sock_proto proto)