tcp: factorize logic into tcp_epollin_ready()

[ Upstream commit 05dc72ab ]

Both tcp_data_ready() and tcp_stream_is_readable() share the same logic.

Add tcp_epollin_ready() helper to avoid duplication.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Arjun Roy <arjunroy@google.com>
Cc: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 2660a544 ("net: Fix TOCTOU issue in sk_is_readable()")
Signed-off-by: Sasha Levin <sashal@kernel.org>