libprocstat: handle race condition while retrieving thread kernel stacks

[wollman]

Summary

When retrieving kernel stacks using sysctl (as for procstat -kk on a running kernel), thread creation can race with allocating the buffer to hold the sysctl results, so the operation fails with [ENOMEM]. As a heuristic, assume that the number of threads (and thus the required buffer size) will at most double during this interval.

Test Plan

I have a machine where this happens somewhat regularly with nfsd and will be installing it there to see if the errors go away completely.