staging:lustre: Fix sparse warnings of Using plain integer as NULL pointer
This patch fixes the following sparse warnings:
drivers/staging/lustre/lustre/lov/lov_obd.c:342:48: warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/lov/lov_obd.c:647:44: warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/lov/lov_obd.c:771:46: warning: Using plain integer as NULL pointer