Reduce the cost of sysinfo(2).

- sysinfo(2) does not actually require a fine-grained breakdown of memory
  usage. Accordingly, instead of calling pgalloc.MemoryFile.UpdateUsage() to
  update the sentry's fine-grained memory accounting snapshot, just use
  pgalloc.MemoryFile.TotalUsage() (which is a single fstat(), and therefore far
  cheaper).

- Use the number of threads in the root PID namespace (i.e. globally) rather
  than in the task's PID namespace for consistency with Linux (which just reads
  global variable nr_threads), and add a new method to kernel.PIDNamespace to
  allow this to be read directly from an underlying map rather than requiring
  the allocation and population of an intermediate slice.

PiperOrigin-RevId: 336353100

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