LU-8066 sysfs: make ping sysfs file read and writable

LU-8066 sysfs: make ping sysfs file read and writable

Starting with 4.15 kernels any sysfs read only is limited to
root access only. To retain the ability for non root users
to detect if a remote server is alive using the 'ping' sysfs
file we need to change it to writable. Retain the read ability
so older tools will work.

Lustre-change: https://review.whamcloud.com/33776
Lustre-commit: 6bbae72c6900dbd2b853d716bc4d456dc7fd586e

Change-Id: I6560c119328d723a20a2b32e1fa8c68dce5d407a
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/33776
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-on: https://review.whamcloud.com/35313
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>