Raise minimum support version

[akiradeveloper]

As of dm-writeboost 2.2.16 it supports older kernels from 3.10 but I don't think 3.10 is still used.

Actually we have a lot of version-switches in the code and this is too ugly.

I want to raise the minimum support version. Then what should it be?

[akiradeveloper]

One of which I want to remove from the codebase is the one with REHL_RELEASE_CODE. Since it is to support quite old kernel, I would remove the support for 4.6 or earlier.