mptcp: allow the in kernel PM to set MPC subflow priority

Any local endpoints configured on the address matching the
MPC subflow are currently ignored.

Specifically, setting a backup flag on them has no effect
on the first subflow, as the MPC handshake can't carry such
info.

This change refactors the MPC endpoint id accounting to
additionally fetch the priority info from the relevant endpoint
and eventually trigger the MP_PRIO handshake as needed.

As a result, the MPC subflow now switches to backup priority
after that the MPTCP socket is fully established, according
to the local endpoint configuration.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>