Staging: lustre: Avoid using 0 instead of NULL

Fixed the following sparse complaints:

drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:755:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:757:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:758:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:759:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:760:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:762:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:764:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:765:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:766:9:
	warning: Using plain integer as NULL pointer
drivers/staging/lustre/lustre/ptlrpc/../../lustre/ldlm/ldlm_pool.c:768:9:
	warning: Using plain integer as NULL pointer

Signed-off-by: Fabio Falzoi <fabio.falzoi84@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>