[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>

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I rebase this patch to the master and resend. Please merge