mptcp: pm: only in-kernel cannot have entries with ID 0

mptcp: pm: only in-kernel cannot have entries with ID 0

The ID 0 is specific per MPTCP connections. The per netns entries cannot
have this special ID 0 then.

But that's different for the userspace PM where the entries are per
connection, they can then use this special ID 0.

Fixes: f40be0db0b76 ("mptcp: unify pm get_flags_and_ifindex_by_id")
Cc: stable@vger.kernel.org
Acked-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240819-net-mptcp-pm-reusing-id-v1-11-38035d40de5b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>