driver core: synchronize device shutdown

A patch series to make .shutdown execute asynchronously.  Some drivers's
shutdown can take a lot of time.  The patches can help save some shutdown
time.  The patches use Arjan's async API.

This patch:

synchronize all tasks submitted by .shutdown

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Dmitry Torokhov <dtor@mail.ru>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>