Implement unr64

Important users of unr like tmpfs or pipes can get away with just
ever-increasing counters, making the overhead of managing the state
for 32 bit counters a pessimization.

Change it to an atomic variable. This can be further sped up by making
the counts variable "allocate" ranges and store them per-cpu.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18054