staging:lustre: Fix sparse warning of plain integer as NULL

This patch fixes the following sparse warning:
drivers/staging/lustre/lustre/lov/lov_request.c:53:28: warning: Using plain integer as NULL pointer

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>