mptcp: factor out a basic skb coalesce helper

The upcoming patch will introduced backlog processing for MPTCP
socket, and we want to leverage coalescing in such data path.

Factor out the relevant bits not touching memory accounting to
deal with such use-case.

Co-developed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250927-net-next-mptcp-rcv-path-imp-v1-6-5da266aa9c1a@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>