When using a swap backed md, round to page_size blocks.

Without this we get EDOM back from mdconfig.
be verbose about mdconfig with swap
Use 1meg chunks to read back the nano image.
Use swap for images, this seems about 10x as fast.
Merge branch 'contrib_fast_md' of https://github.com/alfredperlstein/freenas into alfredperlstein-contrib_fast_md
Fix style introduced in the merge.
Remove debugging code to reduce diff from upstream.