btrfs: send: prepare put_file_data() for large data folios

btrfs: send: prepare put_file_data() for large data folios

Currently put_file_data() can only accept a page sized folio.  However
the function itself is not that complex, it's just copying data from
filemap folio into the send buffer.

Make it support large data folios:

- Change the loop to use file offset instead of page index

- Calculate @pg_offset and @cur_len after getting the folio

- Remove the "WARN_ON(folio_order(folio));" line

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