btrfs: scrub: use virtual addresses directly

Instead of the old @page and @page_offset pair inside scrub, here we can
directly use the virtual address for a sector.

This has the following benefit:

- Simplified parameters
  A single @kaddr will repair @page and @page_offset.

- No more unnecessary kmap/kunmap calls
  Since all pages utilized by scrub is allocated by scrub, and no
  highmem is allowed, we do not need to do any kmap/kunmap.

  And add an ASSERT() inside the new scrub_stripe_get_kaddr() to
  catch any unexpected highmem page.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>