btrfs: compression: migrate compression/decompression paths to folios

For both compression and decompression paths, we always require a
"struct page **pages" and "unsigned long nr_pages", this involves quite
some part of the btrfs compression paths:

- All the compression entry points

- compressed_bio structure
  This affects both compression and decompression.

- async_extent structure

Unfortunately with all those involved parts, there is no good way to
split the conversion into smaller patches while still passing compiling.
So do this in one big conversion in one go.

Please note this is direct page->folio conversion, no change on the page
sized folio requirement yet.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ minor style fixups ]
Signed-off-by: David Sterba <dsterba@suse.com>