Staging: add cowloop driver

Cowloop is a "copy-on-write" pseudo block driver. It can
be stacked on top of a "real" block driver, and catches
all write operations on their way from the file systems
layer above to the real driver below, effectively shielding
the lower driver from those write accesses. The requests are
then diverted to an ordinary file, located somewhere else
(configurable). Later read requests are checked to see whether
they can be serviced by the "real" block driver below, or
must be pulled in from the diverted location. More information
is on the project's website http://www.ATComputing.nl/cowloop/

From: "H.J. Thomassen" <hjt@ATComputing.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>