uvm(9): One rndsource for faults -- not one per CPU.

All relevant state is per-CPU anyway; the only substantive difference
this makes is how many entries appear in `rndctl -l' output and what
they are called -- formerly the somewhat confusing `cpuN', meaning
`page faults on cpuN', and now just `uvmfault'. I don't think
there's any real value in being able to enable or disable measurement
or counting of page faults on one CPU vs others, so although this
could be a minor compatibility change, it's hard to imagine it
matters much.

XXX kernel ABI change in struct cpu_info