[writeboost] move safe_io_wq to inside struct wb_device

Other targets have their own workqueue inside the per-device struct,
not per-module.

This commit moves per-module safe_io_wq to per-device io_wq.
Furthermore, it is renamed to dm- DM_MSG_PREFIX as dm-cache or dm-era
do.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] Move io_client from per-module to per-device

io_client needs to be moved to per-device struct.
Now, module_init/exit only register/unregister target.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge pull request #13 from akiradeveloper/feature/move-global-member

Feature/move global member