LU-9325 mdt: replace simple_strtol() with kstrtol()

LU-9325 mdt: replace simple_strtol() with kstrtol()

Someday simple_strtol() will go away. The simple_strtol() call in
mdt_init0() is very simple so we can easily replace it with
kstrtol().

Change-Id: I37485735f0f42aa5c2c5b9fd361e4fdfa54dc8e5
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/38846
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>