zfs: zstd: track allocator statistics

zfs: zstd: track allocator statistics

This applies:
commit c4ede65bdfca11b532403620bbf0d6e33f0c1c1d
Author: Mateusz Guzik <mjguzik@gmail.com>
Date:   Fri Oct 30 23:26:10 2020 +0100

    zstd: track allocator statistics

    Note that this only tracks sizes as requested by the caller.
    Actual allocated space will almost always be bigger (e.g., rounded up to
    the next power of 2 or page size). Additionally the allocated buffer may
    be holding other areas hostage. Nonetheless, this is a starting point
    for tracking memory usage in zstd.

from openzfs