Use write buffer when writing resulting image

[Gregy]

This modification resulted in large performance gain on my system (5x faster). Reasoning behind this change is that we should not call the kernel with each sector (512b) and instead accumulate more data before sending it.

Please review the change carefully, this is my first encounter with golang and I do not understand it very well (but I used this code to successfully recover my 2T WD drive)

[wolfgang-noichl]

No performance benefit for me, but works.

[maxisvk]

test with Intel G5400 change from ~950Kb/s to ~10Mb/s. Also cpu is always at 100% so with faster cpu gain can be better.