Feature/migrate mb kcopyd

The main purpose of this branch is to use dm_kcopyd_copy() in migrate_mb that was implemented by reading/writing by raw dm_io(). I don't know how this fix affects the performance but will definitely do for readability.

Other two commits include refactoring, comment fixing and optimization.

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I said I fixed the redzone overwritten bug by using dm_kcopyd_copy but it was just a illusion...
It does reproduce even now but other than migrate_mb.