staging: lustre: plain interger was used as NULL pointer

This patch fixes the following warnings found when running sparse:
.../lproc_echo.c:43:11: warning: Using plain integer as NULL pointer
.../lproc_echo.c:49:11: warning: Using plain integer as NULL pointer

Signed-off-by: Josep Puigdemont <josep.puigdemont@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>