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

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>