libceph: fallback for when there isn't a pool-specific choose_arg

There is now a fallback to a choose_arg index of -1 if there isn't
a pool-specific choose_arg set.  If you create a per-pool weight-set,
that works for that pool.  Otherwise we try the compat/default one.  If
that doesn't exist either, then we use the normal CRUSH weights.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>

include/linux/crush/crush.h

@@ -193,7 +193,7 @@ struct crush_choose_arg {
 struct crush_choose_arg_map {
 #ifdef __KERNEL__
 	struct rb_node node;
-	u64 choose_args_index;
+	s64 choose_args_index;
 #endif
 	struct crush_choose_arg *args; /*!< replacement for each bucket
                                             in the crushmap */