uwb: reference count reservations

Reference counting the struct uwb_rsv's is safer and easier to get right than
the transferring ownership of the structures from the PAL to reservation
manager.

This fixes an oops in the debug PAL after a reservation timed out.

Signed-off-by: David Vrabel <david.vrabel@csr.com>

include/linux/uwb.h

@@ -201,6 +201,7 @@ struct uwb_rsv {
 	struct uwb_rc *rc;
 	struct list_head rc_node;
 	struct list_head pal_node;
+	struct kref kref;
 
 	struct uwb_dev *owner;
 	struct uwb_rsv_target target;