lib utils: fix family during af_bit_len calculation

commit f3a2ddc1 ("lib utils: Use helpers to get AF bit/byte len")
used a wrong family or family of zero in the default case
during af_bit_len calculation causing ip route commands to
fail with below error

Error: an inet prefix is expected rather than "10.0.2.14/24".
Reported-by: Sven-Haegar Koch <haegar@sdinet.de>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>

lib/utils.c

@@ -477,7 +477,7 @@ int get_prefix_1(inet_prefix *dst, char *arg, int family)
 
 	err = get_addr_1(dst, arg, family);
 	if (err == 0) {
-		dst->bitlen = af_bit_len(family);
+		dst->bitlen = af_bit_len(dst->family);
 
 		if (slash) {
 			if (get_netmask(&plen, slash+1, 0)