Refactor prefix_adjout_withdraw()

Refactor prefix_adjout_withdraw()

Just pass the prefix to be withdrawn to the function and move the lookup
up. Adjust how the various accounting vars are updated so that the
values are decremented in the right cases. Do the same accounting dance
for prefix_adjout_destroy(). Adjust rde_up_flush_upcall() to directly
call prefix_adjout_withdraw() without calling it via up_generate_updates().
OK tb@