tcp: make tcp_rcvbuf_grow() accessible to mptcp code

To leverage the auto-tuning improvements brought by commit 2da35e4b
("Merge branch 'tcp-receive-side-improvements'"), the MPTCP stack need
to access the mentioned helper.

Acked-by: Geliang Tang <geliang@kernel.org>
Acked-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-v1-2-5da266aa9c1a@kernel.org


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