This started as debugging helper, to watch the effects of converting
between raid levels on multiple devices, but could be useful standalone.

In my case the usage filter was not finegrained enough and led to
converting 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: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NChris Mason <clm@fb.com>