mptcp: fix NL PM announced address accounting

commit 4b317e0eb287bd30a1b329513531157c25e8b692 upstream.

Currently the per connection announced address counter is never
decreased. As a consequence, after connection establishment, if
the NL PM deletes an endpoint and adds a new/different one, no
additional subflow is created for the new endpoint even if the
current limits allow that.

Address the issue properly updating the signaled address counter
every time the NL PM removes such addresses.

Fixes: 01cacb00 ("mptcp: add netlink-based PM")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>