btrfs-progs: balance filter: add limit of processed chunks

Add more control to the balance behaviour.

Usage filter may not be finegrained enough and can lead to moving too
many chunks at once. Another example use is in connection with
drange+devid or vrange filters that allow to work with a specific chunk
or even with a chunk on a given device.

The limit filter applies last, the value of 0 means no limiting.

CC: Ilya Dryomov <idryomov@gmail.com>
CC: Hugo Mills <hugo@carfax.org.uk>
Signed-off-by: David Sterba <dsterba@suse.cz>