mptcp: pm: rename helpers linked to 'flush'

Rename all the helpers specific to the flushing operations to make it
clear that the intention is to flush all created subflows, and remove
all announced addresses, not just a specific selection.

That way, it is easier to understand why the id_avail_bitmap and
local_addr_used are reset at the end.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240902-net-next-mptcp-mib-mpjtx-misc-v1-1-d3e0f3773b90@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>