LU-9731 Limit work-around to EL7 only

LU-9731 Limit work-around to EL7 only

Since the workaround previously landed for LU-9731 only applies to
EL7, only apply it for EL7 builds.

Lustre-change: https://review.whamcloud.com/28202
Lustre-commit: 950c511276866efb6af8defe49213fd69b8883d2

Signed-off-by: Brian J. Murrell <brian.murrell@intel.com>
Change-Id: Id74f03f3af74f324320e094e32f7b7480259145c
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Signed-off-by: Minh Diep <minh.diep@intel.com>
Reviewed-on: https://review.whamcloud.com/28438
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>