mptcp: add 'laminar' endpoint support

mptcp: add 'laminar' endpoint support

This new endpoint type has been recently added to the kernel in v6.18
[1]. It will be used to create new subflows from the associated address
to additional addresses announced by the other peer. This will be done
if allowed by the MPTCP limits, and if the associated address is not
already being used by another subflow from the same MPTCP connection.

Note that the fullmesh flag takes precedence over the laminar one.
Without any of these two flags, the path-manager will create new
subflows to additional addresses announced by the other peer by
selecting the source address from the routing tables, which is harder to
configure if the announced address is not known in advance.

The support of the new flag is easy: simply by adding it in the
mptcp_addr_flag_names array.

The usage menu and the manual now references the new endpoint type. The
new corresponding counter has also been added in ss.

Link: https://git.kernel.org/torvalds/c/539f6b9de39e
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David Ahern <dsahern@kernel.org>