Accept device name "cancel" as a request to cancel running device
deletion operation. The string is literal, in case there's a real device
named "cancel", pass it as full absolute path or as "./cancel"

This works for v1 and v2 ioctls when the device is specified by name.
Moving chunks from the device uses relocation, use the conditional
exclusive operation start and cancellation helpers
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>