btrfs: open code folio_index() in btree_clear_folio_dirty_tag()

The folio_index() helper is only needed for mixed usage of page cache
and swap cache, for pure page cache usage, the caller can just use
folio->index instead.

It can't be a swap cache folio here.  Swap mapping may only call into fs
through 'swap_rw' but btrfs does not use that method for swap.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>