staging: lustre: use module_name() macro for debug printf

We now allow lustre to be built when CONFIG_MODULES is disabled,
but that causes a build failure:

In file included from drivers/staging/lustre/include/linux/libcfs/libcfs.h:42,
                 from drivers/staging/lustre/lustre/obdclass/lu_object.c:44:
drivers/staging/lustre/lustre/obdclass/lu_object.c: In function 'lu_context_key_degister':
drivers/staging/lustre/lustre/obdclass/lu_object.c:1410:51: error: dereferencing pointer to incomplete type 'struct module'

This particular case can be avoided by using the module_name()
macro that was designed exactly to handle printing the name of
a module in all configurations.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>