mptcp: more careful RM_ADDR generation

The in-kernel MPTCP path manager, when processing the MPTCP_PM_CMD_FLUSH_ADDR
command, generates RM_ADDR events for each known local address. While that
is allowed by the RFC, it makes unpredictable the exact number of RM_ADDR
generated when both ends flush the PM addresses.

This change restricts the RM_ADDR generation to previously explicitly
announced addresses, and adjust the expected results in a bunch of related
self-tests.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>