bpf: fix the check that forwarding is enabled in bpf_ipv6_fib_lookup

The bpf_ipv6_fib_lookup function should return BPF_FIB_LKUP_RET_FWD_DISABLED
when forwarding is disabled for the input device.  However instead of checking
if forwarding is enabled on the input device, it checked the global
net->ipv6.devconf_all->forwarding flag.  Change it to behave as expected.

Fixes: 87f5fc7e ("bpf: Provide helper to do forwarding lookups in kernel FIB table")
Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>