MAINTAINERS: add Geliang as reviewer for MPTCP

For a long time now, Geliang has contributed to a lot of code and
reviews related to MPTCP. So let's reflect that in the MAINTAINERS file.

This should also encourage patch submitters to add him to the CC list.

Acked-by: Geliang Tang <geliang.tang@linux.dev>
Acked-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20231226-upstream-net-20231226-mptcp-prevent-warn-v1-1-1404dcc431ea@kernel.org


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