netlink: add nla_get_s32()

This function was added into libnl3 recently,
but we have to support old versions of this library.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/include/libnetlink.h

@@ -11,6 +11,8 @@ extern int do_rtnl_req(int nl, void *req, int size,
 extern int addattr_l(struct nlmsghdr *n, int maxlen, int type,
 		const void *data, int alen);
 
+extern int32_t nla_get_s32(const struct nlattr *nla);
+
 #define NLMSG_TAIL(nmsg) \
 	((struct rtattr *) (((void *) (nmsg)) + NLMSG_ALIGN((nmsg)->nlmsg_len)))
 

criu/libnetlink.c

@@ -219,3 +219,8 @@ int __wrap_nlmsg_parse(struct nlmsghdr *nlh, int hdrlen, struct nlattr *tb[],
 	return nla_parse(tb, maxtype, nlmsg_attrdata(nlh, hdrlen),
 			 nlmsg_attrlen(nlh, hdrlen), policy);
 }
+
+int32_t nla_get_s32(const struct nlattr *nla)
+{
+        return *(const int32_t *) nla_data(nla);
+}