[writeboost] fix: where to clean up cache

activate() first activates the underlying devices and then activates the
wb device with them.
cleanup_cache (zeros out the superblock) needs to be called right after
the first activation, which is of underlying devices. not the second
one.

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

[writeboost] fix: where to clean up cache