staging: writeboost: Add dm-writeboost

This patch adds dm-writeboost to staging tree.

dm-writeboost is a log-structured SSD-caching driver.
It caches data in log-structured way on the cache device
so that the performance is maximized.

The merit of putting this driver in staging tree is
to make it possible to get more feedback from users
and polish the codes.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
staging: writeboost: Add read caching

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
update

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
update

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
update

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
update (STATUS: compiled)

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>