bcache: asynchronous devices registration

This simple series is to add an asynchronous devices registration
interface for experiment. The async registration may help the bcache
register udev task to return without wait, and avoid to be killed
due to the udevd timeout.

For common users this async interface is unnecessary. For users who
has extream large cached data set and encounters the boot time bcache
registration failure may have a try on this series. This asynchronous
registration behavior will be set to default in future.

Thanks in advance for your comments and feedback.

Coly Li
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Coly Li (2):
  bcache: asynchronous devices registration
  bcache: configure the asynchronous registertion to be experimental

 drivers/md/bcache/Kconfig |   9 ++++
 drivers/md/bcache/super.c | 102 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 111 insertions(+)

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2.25.0