pimd: Clean back pointer upon deletion

pimd: Clean back pointer upon deletion

The channel_oil has a back pointer(up) to the upstream data structure.
If we are planning on keeping the channel oil (due to ref count issues)
longer than keeping the upstream, when we delete the upstream we were
not clearing the back pointer to up.  This would result in a situation
where if that memory has started to be used again it will cause a
crash and other fun things.

Ticket: CM-17092
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>