VM subsystem twice for every written record.
Tests on 24-core system show double reduction of CPU time spent on copying
single large well-compressed file.
This patch is not really needed on illumos (while not harm either) since
their memory allocator by default uses caching for all requests up to 128K.
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
e5056f9882