bcache: Incremental gc

Big garbage collection rewrite; now, garbage collection uses the same
mechanisms as used elsewhere for inserting/updating btree node pointers,
instead of rewriting interior btree nodes in place.

This makes the code significantly cleaner and less fragile, and means we
can now make garbage collection incremental - it doesn't have to hold a
write lock on the root of the btree for the entire duration of garbage
collection.

This means that there's less of a latency hit for doing garbage
collection, which means we can gc more frequently (and do a better job
of reclaiming from the cache), and we can coalesce across more btree
nodes (improving our space efficiency).

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/sysfs.c

@@ -489,7 +489,6 @@ lock_root:
 
 	sysfs_print(btree_used_percent,	btree_used(c));
 	sysfs_print(btree_nodes,	c->gc_stats.nodes);
-	sysfs_hprint(dirty_data,	c->gc_stats.dirty);
 	sysfs_hprint(average_key_size,	average_key_size(c));
 
 	sysfs_print(cache_read_races,
@@ -642,7 +641,6 @@ static struct attribute *bch_cache_set_files[] = {
 	&sysfs_cache_available_percent,
 
 	&sysfs_average_key_size,
-	&sysfs_dirty_data,
 
 	&sysfs_errors,
 	&sysfs_io_error_limit,