The initial subflow has a special local ID: 0. It is specific per
connection.

When a global endpoint is deleted and re-added later, it can have a
different ID -- most services managing the endpoints automatically don't
force the ID to be the same as before. It is then important to track
these modifications to be consistent with the ID being used for the
address used by the initial subflow, not to confuse the other peer or to
send the ID 0 for the wrong address.

Now when removing an endpoint, msk->mpc_endpoint_id is reset if it
corresponds to this endpoint. When adding a new endpoint, the same
variable is updated if the address match the one of the initial subflow.

Fixes: 3ad14f54 ("mptcp: more accurate MPC endpoint tracking")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>