Staging: lustre: lnet: Declare ping_srv_workitems static

This patch makes ping_srv_workitems static as it is
used only in the file ping_test.c

This problem is detected by sparse.

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>