btrfs: raid56: use list_last_entry() at cache_rbio()

Instead of using list_entry() against the list's prev entry, use
list_last_entry(), which removes the need to know the last member is
accessed through the prev list pointer and the naming makes it easier
to reason about what we are doing.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/raid56.c

@@ -577,9 +577,9 @@ static void cache_rbio(struct btrfs_raid_bio *rbio)
 	if (table->cache_size > RBIO_CACHE_SIZE) {
 		struct btrfs_raid_bio *found;
 
-		found = list_entry(table->stripe_cache.prev,
-				  struct btrfs_raid_bio,
-				  stripe_cache);
+		found = list_last_entry(&table->stripe_cache,
+					struct btrfs_raid_bio,
+					stripe_cache);
 
 		if (found != rbio)
 			__remove_rbio_from_cache(found);