nft: replace nft_rule_attr_get_u8

Since the family declaration has been modified in libnftnl,
from commit 3cd9cd06625f8181c713489cec2c1ce6722a7e16
the assertion is failed for {ip,ip6,arp}tables-compat
when printing rules.

iptables-compat -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
libnftnl: attribute 0 assertion failed in rule.c:273

ip6tables-compat -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
libnftnl: attribute 0 assertion failed in rule.c:273

arptables-compat -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
libnftnl: attribute 0 assertion failed in rule.c:273

Signed-off-by: Giuseppe Longo <giuseppelng@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>