btrfs: sysfs: export the balance paused state of exclusive operation

The new state allowing device addition with paused balance is not
exported to user space so it can't recognize it and actually start the
operation.

Fixes: efc0e69c ("btrfs: introduce exclusive operation BALANCE_PAUSED state")
CC: stable@vger.kernel.org # 5.17
Signed-off-by: David Sterba <dsterba@suse.com>