pimd: Convert pim_ifchannel_list to use up->ifchannels

pimd: Convert pim_ifchannel_list to use up->ifchannels

We have a bunch of places where we iterate over
the pim_ifchannel_list to find those ifchannels
that match a certain upstream.  Since we already
know in the upstream the list of ifchannels
associated with it, just look at those instead.

Functions changed:

forward_on
forward_off
pim_upstream_rpf_interface_changed
pim_upstream_update_could_assert
pim_upstream_update_my_assert_metric
pim_upstream_update_assert_tracking_desired
pim_upstream_is_sg_rpt

Ticket: CM-15629
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>