LU-13039 quota: Ensure local buffer is null terminated

LU-13039 quota: Ensure local buffer is null terminated

Found via KASAN, copy_from_user may not set null
terminator.

Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I20de911c2b2d50a1715a27e3edfe65442eaa2be6
Reviewed-on: https://review.whamcloud.com/36899
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>
Reviewed-by: Sergey Cheremencev <c17829@cray.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>