mptcp: sockopt: increase seq in mptcp_setsockopt_all_sf

mptcp: sockopt: increase seq in mptcp_setsockopt_all_sf

mptcp_setsockopt_all_sf() was missing a call to sockopt_seq_inc(). This
is required not to cause missing synchronization for newer subflows
created later on.

This helper is called each time a socket option is set on subflows, and
future ones will need to inherit this option after their creation.

Fixes: 51c5fd09e1b4 ("mptcp: add TCP_MAXSEG sockopt support")
Cc: stable@vger.kernel.org
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260501-net-mptcp-misc-fixes-7-1-rc3-v1-4-b70118df778e@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>