btrfs: expand subpage support to any PAGE_SIZE > 4K

With the recent change in metadata handling, we can handle metadata in
the following cases:

- nodesize < PAGE_SIZE and sectorsize < PAGE_SIZE
  Go subpage routine for both metadata and data.

- nodesize < PAGE_SIZE and sectorsize >= PAGE_SIZE
  Invalid case for now. As we require nodesize >= sectorsize.

- nodesize >= PAGE_SIZE and sectorsize < PAGE_SIZE
  Go subpage routine for data, but regular page routine for metadata.

- nodesize >= PAGE_SIZE and sectorsize >= PAGE_SIZE
  Go regular page routine for both metadata and data.

Now we can handle any sectorsize < PAGE_SIZE, plus the existing
sectorsize == PAGE_SIZE support.

But here we introduce an artificial limit, any PAGE_SIZE > 4K case, we
will only support 4K and PAGE_SIZE as sector size.

The idea here is to reduce the test combinations, and push 4K as the
default standard in the future.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>