[writeboost] Remove term migrate

Now writeboost uses two terms "migrate" and "writeback" to
represent writeback. The term "writeback" is more intuitive so
we don't use the term "migrate" anymore.

This is a patch that must be merged atomically with
one to the kernel.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>

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This is a counterpart of the pull request to your linux-2.6.