LU-9325 obdclass: make niduuid for lustre_stop_mgc() static

LU-9325 obdclass: make niduuid for lustre_stop_mgc() static

The process to create a proper string for niduuid can be made
simpler and avoid a memory allocation.

Change-Id: I52cb01117e41cbcf2756477e91934a42d31fd157
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/33617
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>