Staging: android: binder: Create dedicated workqueue for binder deferred work

Some drivers flush the global workqueue when closed. This would deadlock if
the last reference to the file was released from the binder.

Signed-off-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>