btrfs: support all block sizes which is no larger than page size

btrfs: support all block sizes which is no larger than page size

Currently if block size < page size, btrfs only supports one single
config, 4K.

This is mostly to reduce the test configurations, as 4K is going to be
the default block size for all architectures.

However all other major filesystems have no artificial limits on the
support block size, and some are already supporting block size > page
sizes.

Since the btrfs subpage block support has been there for a long time,
it's time for us to enable all block size <= page size support.

So here enable all block sizes support as long as it's no larger than
page size for experimental builds.

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