userspace: rtu_stat: fix output format

Output for the mask parameter was wrong if high bits are zero.
Not exposed by default (mask is 0xffffffff).

But I got

     VID    FID       MASK       DROP    PRIO    PRIO_OVERRIDE
    102    102      0x   7ffff    NO      --         NO

Which now is

    102    102      0x0007ffff    NO      --         NO
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>