gfs2: Simplify do_promote

While not immediately obvious, do_promote() returns whether or not there
are any active holders in the queue.  But the function description is
confusing, and this information is easy to come by for callers anyway,
so turn do_promote() into a void function.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>