Increase pm subflows counter on both server side and client side when
userspace pm creates a new subflow, and decrease the counter when it
closes a subflow.

Increase add_addr_signaled counter in mptcp_nl_cmd_announce() when the
address is announced by userspace PM.

This modification is similar to how the in-kernel PM is updating the
counter: when additional subflows are created/removed.

Fixes: 9ab4807c ("mptcp: netlink: Add MPTCP_PM_CMD_ANNOUNCE")
Fixes: 702c2f64 ("mptcp: netlink: allow userspace-driven subflow establishment")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/329
Cc: stable@vger.kernel.org
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>