mptcp: introduce add_addr_v6_port_drop_ts sysctl knob

mptcp: introduce add_addr_v6_port_drop_ts sysctl knob

This sysctl is going to be used in the next commits to drop TCP
timestamps option, to be able to send an ADD_ADDR with a v6 IP address
and a port number. It is enabled by default.

This knob is explicitly disabled in the MPTCP Join selftest, with the
"signal addr list progresses after tx drop" subtest, to continue
verifying the previous behaviour where the ADD_ADDR is not sent due to a
lack of space.

While at it, move syn_retrans_before_tcp_fallback down from struct
mptcp_pernet, to avoid creating another 3 bytes hole.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260605-net-next-mptcp-add-addr6-port-ts-v2-4-758e7ca73f4d@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>