LU-7334 lprocfs: Refactored string to value helpers

LU-7334 lprocfs: Refactored string to value helpers

Refactored the string parsing helpers to correct
inconsistent behavior and added handling for s64
overflow/underflow. Removed the old helper
functions and refactored functions that called
them.

Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Change-Id: I84f3b5d46d4e20bd09fe223589e1be96dc8f7468
Reviewed-on: http://review.whamcloud.com/16930
Reviewed-by: Christopher J. Morrone <morrone2@llnl.gov>
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>