btrfs: factor out metadata subpage detection into a dedicated helper

btrfs: factor out metadata subpage detection into a dedicated helper

Currently we have only one btrfs_is_subpage() to cover both data and
metadata.

But there is a special case for metadata:

- dummy extent buffer, sector size < PAGE_SIZE and node size >= PAGE_SIZE

In such case, btrfs_is_subpage() will return true for extent buffer
folio.

But that is not correct, and that's exactly why we have some open-coded
checks for functions like set_extent_buffer_uptodate() and
clear_extent_buffer_uptodate().

Just extract the metadata specific checks into a helper, and replace
those call sites.

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