tipc: remove leaving group member from all lists

A group member going into state LEAVING should never go back to any
other state before it is finally deleted. However, this might happen
if the socket needs to send out a RECLAIM message during this interval.
Since we forget to remove the leaving member from the group's 'active'
or 'pending' list, the member might be selected for reclaiming, change
state to RECLAIMING, and get stuck in this state instead of being
deleted. This might lead to suppression of the expected 'member down'
event to the receiver.

We fix this by removing the member from all lists, except the RB tree,
at the moment it goes into state LEAVING.

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>