LU-8767 llite: Improve proc file text in lproc_llite.c

LU-8767 llite: Improve proc file text in lproc_llite.c

Improves the instructions displayed when reading from the stats
files.  Several repeated code blocks are consolidated into a new
function to reduce duplication.

Signed-off-by: Steve Guminski <stephenx.guminski@intel.com>
Change-Id: Ic04ca76016e6e6568ea9f81a8af6153d99053412
Reviewed-on: https://review.whamcloud.com/23942
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>