[writeboost] modified comments of migrate_buffered_mb()

The comment is obsolete. Therefore, fix

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] use dm_kcopyd_copy in migrate_mb

migrate_mb should use dm_kcopyd_copy to show clearly it copies data from cache
device to the backing device.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] delete submit_flush_request from migrate_mb

We don't need to make the data migrated in this function persistent
because migrate daemon will do it later and this segment will never be
reused until then.

Comments are fixed.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge pull request #5 from akiradeveloper/feature/migrate-mb-kcopyd

Feature/migrate mb kcopyd