LU-9176 osd-zfs: improve statfs estimate for ZFS

LU-9176 osd-zfs: improve statfs estimate for ZFS

When a ZFS filesystem is newly formatted, there are 128 OI ZAPs and
96 object directories, but only a handful of regular files.  This
skews the average MDT file size to be much larger than regular files,
which in turn makes the estimated total number of files on the MDT
much too small.

Improve the inode size estimate when the filesystem has few files to
be closer to the expected long term average so that the reported total
inode count is reasonable for a new or otherwise empty filesystem.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ic40af2057d8b21e0ee4d9bb3b13b7f6786d0d406
Reviewed-on: https://review.whamcloud.com/25743
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>