slow-work: use get_ref wrapper instead of directly calling get_ref

Otherwise we can get an oops if the user has no get_ref/put_ref
requirement.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>