btrfs: move folio initialization to one place in attach_eb_folio_to_filemap()

This is just a trivial change. The code looks a bit more readable this way, IMO.

Move initialization of existing_folio to the beginning of the retry loop
so it's set to NULL at one place.

Signed-off-by: Daniel Vacek <neelx@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>