Remove an extra if_ref().

In r348254, if_snd_tag_alloc() routines were changed to bump the ifp
refcount via m_snd_tag_init().  This function wasn't in the tree at
the time and wasn't updated for the new semantics, so was still doing
a separate bump after if_snd_tag_alloc() returned.

Reviewed by:	gallatin
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D26999

sys/netinet/tcp_ratelimit.c

@@ -470,7 +470,6 @@ rl_attach_txrtlmt(struct ifnet *ifp,
 		error = ifp->if_snd_tag_alloc(ifp, &params, tag);
 #ifdef INET
 		if (error == 0) {
-			if_ref((*tag)->ifp);
 			counter_u64_add(rate_limit_set_ok, 1);
 			counter_u64_add(rate_limit_active, 1);
 		} else