utils: make rt_addr_n2a() non-reentrant by default

There is only a single user who needs it to be reentrant (not really,
but it's safer like this), add rt_addr_n2a_r() for it to use.

Signed-off-by: Phil Sutter <phil@nwl.cc>
diff --git a/ip/ip6tunnel.c b/ip/ip6tunnel.c
index d588645..c02fa07 100644
--- a/ip/ip6tunnel.c
+++ b/ip/ip6tunnel.c
@@ -78,7 +78,7 @@
 	       p->name,
 	       tnl_strproto(p->proto),
 	       format_host_r(AF_INET6, 16, &p->raddr, s1, sizeof(s1)),
-	       rt_addr_n2a(AF_INET6, 16, &p->laddr, s2, sizeof(s2)));
+	       rt_addr_n2a_r(AF_INET6, 16, &p->laddr, s2, sizeof(s2)));
 	if (p->link) {
 		const char *n = ll_index_to_name(p->link);