LU-11176 systemd: use univeral path for modprobe

LU-11176 systemd: use univeral path for modprobe

The program modprobe is not the same on all platforms. On RHEL
systems it is located in /usr/sbin. For Ubuntu/Debian which is
busybox based /sbin/modprobe is a symlink to /bin/kmod. On all
platforms to keep some sort of standard a symlink for modprobe
exist in /sbin. Update the lnet.service script to use the hard
patch /sbin/modprobe

Test-Parameters: trivial

Lustre-change: https://review.whamcloud.com/32520
Lustre-commit: 574e63fc86553510d87d02cd6d72785f341e48dc

Change-Id: I54342971a6ee1aa4ce86a9fae0ac4dcb167b1510
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/32944
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/32970
Reviewed-by: Thomas Stibor <t.stibor@gsi.de>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>