mptcp: consolidate passive msk socket initialization

When the msk socket is cloned at MPC handshake time, a few
fields are initialized in a racy way outside mptcp_sk_clone()
and the msk socket lock.

The above is due historical reasons: before commit a88d0092
("mptcp: simplify subflow_syn_recv_sock()") as the first subflow socket
carrying all the needed date was not available yet at msk creation
time

We can now refactor the code moving the missing initialization bit
under the socket lock, removing the init race and avoiding some
code duplication.

This will also simplify the next patch, as all msk->first write
access are now under the msk socket lock.

Fixes: 0397c6d8

 ("mptcp: keep unaccepted MPC subflow into join list")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>