Fix sha256 for scatter abd

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Rename some ABD api

The "linear" thing in the original api sometimes refers to linear type ABD,
buf also refers to normal buffer. This might be confusing. This patch changes
the naming so that "linear" always refers to linear type ABD while "buf"
refers to normal buffers.

I have check that the objdumps does not change with this patch.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Remove unreached code

It seems there aren't any scatter ABD going in those parts. And it's actually
quite natural, because those are for metadata, and metadata should be all
linear.

Signed-off-by: Chunwei Chen <tuxoko@gmail.com>