udevd: fix serialization of events

udevd: fix serialization of events

We see sequences like:
  UEVENT[1176966403.907336] add@/block/dasda
  UDEV  [1176966403.920359] add@/block/dasda
  UEVENT[1176966403.964379] add@/block/dasda/dasda1
  UEVENT[1176966403.964438] add@/block/dasda/dasda2
  UEVENT[1176966403.964483] online@/block/dasda
  UEVENT[1176966403.964519] online@/block/dasda/dasda1
  UEVENT[1176966403.964555] online@/block/dasda/dasda2
  UDEV  [1176966403.967413] add@/block/dasda/dasda1
  UDEV  [1176966403.968719] add@/block/dasda/dasda2
  UDEV  [1176966403.974750] online@/block/dasda/dasda1
  UDEV  [1176966403.982241] online@/block/dasda/dasda2
  UDEV  [1176966403.999847] online@/block/dasda

where the event for the disk (last event) should come before the
events for the partitons. We skipped events for dependency reasons,
but the dependency check for later events did not see the skipped
events.