pctrie: use popmap in locked lookup_range

When lookup_range is invoked with a lock held, the popmap field of a
level-0 parent node can be used to find the next null child (if any)
of that parent, allowing null checks to be removed from the loop and
for the compiler to perform some loop unrolling.

Reviewed by: alc, kib
Differential Revision: https://reviews.freebsd.org/D50389