LU-10444 utils: Don't remount debugfs every time

LU-10444 utils: Don't remount debugfs every time

Check if debugfs is mounted at /sys/kernel/debug and only
mount if it is not.

Lustre-change: https://review.whamcloud.com/30675
Lustre-commit: 767f03b161ae44bd9d33dae7e03e71e73852813f

Change-Id: Ib31bd8f7c5c93ab942c6708ed3a4d17a11159e95
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/30900
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>