LU-5240 obdclass: Increase minimum LU Cache for ZFS

If the lu cache is too small threads will thrash for data including
the OI tables and other important metadata.

Test-Parameters: mdsfilesystemtype=zfs mdtfilesystemtype=zfs ostfilesystemtype=zfs testgroup=review-zfs
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I1ce7ef9a160c8b028be0d3c9671a0b347cbe3331
Reviewed-on: http://review.whamcloud.com/10802
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Tested-by: Jenkins