net: core: add READ_ONCE/WRITE_ONCE annotations for sk->sk_bound_dev_if

sock_bindtoindex_locked() needs to use WRITE_ONCE(sk->sk_bound_dev_if, val),
because other cpus/threads might locklessly read this field.

sock_getbindtodevice(), sock_getsockopt() need READ_ONCE()
because they run without socket lock held.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>