staging: lustre: lov: Fix sparse warning using plain integer as NULL pointer

This patch fixes the warning generated by sparse: "Using plain integer as NULL pointer" by replacing the offending 0s with NULL.

drivers/staging/lustre/lustre/lov/lov_obd.c:902:48: warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/lov/lov_obd.c:946:54: warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/lov/lov_obd.c:2819:46: warning: Using plain integer as NULL pointer

Signed-off-by: Marc Fite <marc@fite.cat>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>