btrfs: convert btrfs_compr_pool_scan() to use folios

btrfs: convert btrfs_compr_pool_scan() to use folios

The compression pool holds order-0 folios, but btrfs_compr_pool_scan()
walks it as struct page through page->lru. Walk it as folios, matching
the other compression pool functions. This removes the last use of
page->lru in btrfs and saves a call to compound_head() per freed
folio.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>