mptcp: pm: change to fullmesh only for 'subflow'

mptcp: pm: change to fullmesh only for 'subflow'

If an endpoint doesn't have the 'subflow' flag -- in fact, has no type,
so not 'subflow', 'signal', nor 'implicit' -- there are then no subflows
created from this local endpoint to at least the initial destination
address. In this case, no need to call mptcp_pm_nl_fullmesh() which is
there to recreate the subflows to reflect the new value of the fullmesh
attribute.

Similarly, there is then no need to iterate over all connections to do
nothing, if only the 'fullmesh' flag has been changed, and the endpoint
doesn't have the 'subflow' one. So stop early when dealing with this
specific case.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250221-net-next-mptcp-pm-misc-cleanup-3-v1-2-2b70ab1cee79@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>