btrfs: convert btree_write_cache_pages() to use filemap_get_folio_tag()

Convert function to use folios throughout.  This is in preparation for the
removal of find_get_pages_range_tag().

Link: https://lkml.kernel.org/r/20230104211448.4804-7-vishal.moola@gmail.com


Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Acked-by: David Sterba <dsterba@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>