btrfs: convert get_next_extent_buffer() to take a folio

The old page API is being gradually replaced and converted to use folio
to improve code readability and avoid repeated conversion between page
and folio. Use folio_pos instead of page_offset, which is more
consistent with folio usage.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>