carp: define CARP_AUTHLEN for carp_authlen field

Replace the bare 7 with a named constant.

sys/netinet/ip_carp.c

@@ -1216,7 +1216,7 @@ carp_send_ad_locked(struct carp_softc *sc)
 	ch.carp_vhid = sc->sc_vhid;
 	ch.carp_advbase = sc->sc_advbase;
 	ch.carp_advskew = advskew;
-	ch.carp_authlen = 7;	/* XXX DEFINE */
+	ch.carp_authlen = CARP_AUTHLEN;
 	ch.carp_pad1 = 0;	/* must be zero */
 	ch.carp_cksum = 0;
 

sys/netinet/ip_carp.h

@@ -80,6 +80,12 @@ struct carp_header {
 
 CTASSERT(sizeof(struct carp_header) == 36);
 
+/*
+ * CARP authentication length in 32-bit chunks:
+ * counter[2] (8 bytes) + SHA1 HMAC (20 bytes) = 28 bytes = 7 chunks.
+ */
+#define	CARP_AUTHLEN	7
+
 /*
  * The VRRPv3 header layout is as follows:
  * See RFC9568, 5.1.  VRRP Packet Format