LU-12726 mdt: Fix usage of sscanf

LU-12726 mdt: Fix usage of sscanf

sscanf is returning the number of items matched in the
input, but we need to return the amount of data
successfully written.

Fixes: a408e9dd426f ("LU-8066 mdt: migrate procfs files to sysfs")

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ib90bd260e745692d11656d0b74820573aaa35550
Reviewed-on: https://review.whamcloud.com/36052
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>