dbus: Fix flush_object_timeout_handler context to be consistent

dbus: Fix flush_object_timeout_handler context to be consistent

The path pointer used as the timeout_ctx was not constant; the path
string itself may have been the same, but the pointer certainly was not
since it was sometimes from stack and sometimes from the dynamically
allocated buffer in obj_desc. This caused some of the eloop timeout
cancellations not to find the timeout. Fix this by using the obj_desc
as the timeout context data.