Merge branch 'mptcp-pm-netlink-announce-server-side-flag'

Matthieu Baerts says:

====================
mptcp: pm: netlink: announce server-side flag

Now that the 'flags' attribute is used, it seems interesting to add one
flag for 'server-side', a boolean value.

Here are a few patches related to the 'server-side' attribute:

- Patch 1: only announce this attribute on the server side.

- Patch 2: announce the 'server-side' flag when this is the case.

- Patch 3: deprecate the 'server-side' attribute.

- Patch 4: use the 'server-side' flag in the selftests.

- Patches 5, 6: small cleanups when working on code around.
====================

Link: https://patch.msgid.link/20250919-net-next-mptcp-server-side-flag-v1-0-a97a5d561a8b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>