ubifs: Simplify the return expression of run_gc()

Simplify the return expression.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Richard Weinberger <richard@nod.at>