mptcp: annotate lockless accesses to sk->sk_err

[ Upstream commit 9ae8e5ad ]

mptcp_poll() reads sk->sk_err without socket lock held/owned.

Add READ_ONCE() and WRITE_ONCE() to avoid load/store tearing.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: d5fbeff1 ("mptcp: move __mptcp_error_report in protocol.c")
Signed-off-by: Sasha Levin <sashal@kernel.org>