LU-6838 llog: limit file size of plain logs

on small filesystems plain log can grow dramatically. especially
given large record sizes produced by DNE and extended chunksize.
I saw >50% of space consumed by a single llog file which was still
in use. this leads to test failures (sanityn, etc).
the patch introduces additional limit on plain llog size, which
is calculated as <free space>/64 (128MB at most) at llog creation
time.

Change-Id: I0eab8177d4e416a32a6aab56d47e4142c81d13de
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: https://review.whamcloud.com/18028
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>