staging: lustre: llite: make default_easize writeable in /sysfs

Allow default_easize to be tuned via /sysfs. A system administrator
might want this if a rare access to widely striped files drives up the
value on a filesystem where narrowly striped files are the more common
case. In practice, however, this is wanted primarily to facilitate
a test case for LU-5549.

 - Plumb the necessary interfaces through the LMV and MDC layers
   to expose write access to this value by higher layers.

 - Add block comments to modified functions.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5549
Reviewed-on: http://review.whamcloud.com/13112


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>