tcp: export tcp_splice_state

tcp: export tcp_splice_state

Export struct tcp_splice_state and tcp_splice_data_recv() in net/tcp.h
so that they can be used by MPTCP in the next patch.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Acked-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260130-net-next-mptcp-splice-v2-3-31332ba70d7f@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>