net/ipv6: Make __inet6_bind static

BPF core gets access to __inet6_bind via ipv6_bpf_stub_impl, so it is
not invoked directly outside of af_inet6.c. Make it static and move
inet6_bind after to avoid forward declaration.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>