btrfs: rename btrfs_subpage structure

btrfs: rename btrfs_subpage structure

With the incoming large data folios support, the structure name
btrfs_subpage is no longer correct, as for we can have multiple blocks
inside a large folio, and the block size is still page size.

So to follow the schema of iomap, rename btrfs_subpage to
btrfs_folio_state, along with involved enums.

There are still exported functions with "btrfs_subpage_" prefix, and I
believe for metadata the name "subpage" will stay forever as we will
never allocate a folio larger than nodesize anyway.

The full cleanup of the word "subpage" will happen in much smaller steps
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>