LU-15642 obdclass: use consistent stats units

LU-15642 obdclass: use consistent stats units

Use consistent stats units, since some were "usec" and others "usecs".
Most stats already use LPROCFS_TYPE_* to encode type stats type, so
use this to provide units for those stats, and only explicitly provide
strings for the few stats that don't match the commonly-used units.
This also reduces the number of repeat static strings in the modules.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I25f31478f238072ddbf9a3918cd43bb08c3ebbe5
Reviewed-on: https://review.whamcloud.com/46833
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Ben Evans <beevans@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>