net: annotate data-races around sk->sk_reserved_mem

sk_getsockopt() runs locklessly. This means sk->sk_reserved_mem
can be read while other threads are changing its value.

Add missing annotations where they are needed.

Fixes: 2bb2f5fb

 ("net: add new socket option SO_RESERVE_MEM")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>