staging: lustre: obdclass: Fix comparison to NULL

Replace comparison to NULL with a 'not' operator.
Issue found with checkpatch.

Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>