bpf: Change bpf_fib_lookup to return -EAFNOSUPPORT for unsupported address families

Update bpf_fib_lookup to return -EAFNOSUPPORT for unsupported address
families. Allows userspace to probe for support as more are added
(e.g., AF_MPLS).

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>