staging: lustre: Fix sparse, using plain integer as NULL pointer in lov_object_fiemap()

Change 0 to NULL in lov_object_fiemap() in order to fix warning produced by
sparse

Signed-off-by: Andrii Vladyka <tulup@mail.ru>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>