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>

fs/btrfs/sysfs.c

@@ -922,6 +922,9 @@ static ssize_t btrfs_exclusive_operation_show(struct kobject *kobj,
 		case BTRFS_EXCLOP_BALANCE:
 			str = "balance\n";
 			break;
+		case BTRFS_EXCLOP_BALANCE_PAUSED:
+			str = "balance paused\n";
+			break;
 		case BTRFS_EXCLOP_DEV_ADD:
 			str = "device add\n";
 			break;