net: annotate sk->sk_err write from do_recvmmsg()

do_recvmmsg() can write to sk->sk_err from multiple threads.

As said before, many other points reading or writing sk_err
need annotations.

Fixes: 34b88a68 ("net: Fix use after free in the recvmmsg exit path")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>