LU-5549 llite: make default_easize writeable in /proc

Allow default_easize to be tuned via /proc. 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.

- Correct misspelling of "default" in /proc handler function names
  in lustre/llite/lproc_llite.c. The file names in /proc were already
  spelled correctly so there are no issues with backward
  compatibility.

- Convert remaining space-indented lines in lmv_set_info_async()
  to tabs.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Change-Id: Iae2c8d0ca28cccf12af9372b1a10a0f9d170fddf
Reviewed-on: http://review.whamcloud.com/13112
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>