trace: mptcp: add mptcp_rcvbuf_grow tracepoint

trace: mptcp: add mptcp_rcvbuf_grow tracepoint

Similar to tcp, provide a new tracepoint to better understand
mptcp_rcv_space_adjust() behavior, which presents many artifacts.

Note that the used format string is so long that I preferred
wrap it, contrary to guidance for quoted strings.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260203-net-next-mptcp-misc-feat-6-20-v1-4-31ec8bfc56d1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>