mptcp: grafting MPJ subflow earlier

mptcp: grafting MPJ subflow earlier

Later patches need to ensure that all MPJ subflows are grafted to the
msk socket before accept() completion.

Currently the grafting happens under the msk socket lock: potentially
at msk release_cb time which make satisfying the above condition a bit
tricky.

Move the MPJ subflow grafting earlier, under the msk data lock, so that
we can use such lock as a synchronization point.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251121-net-next-mptcp-memcg-backlog-imp-v1-3-1f34b6c1e0b1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>