LU-7623 lnet: Properly declare lnet_ping() forward declaration

LU-7623 lnet: Properly declare lnet_ping() forward declaration

It was missing __user attribute for the userspace pointer before.

Change-Id: I21937c153294387f14e3114d7970dc879a055cbe
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/17784
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>