userspace: rtu_stat: fix output format

Output for the mask parameter was wrong if high bits are zero.
This is not usually noticed as by default the mask is 0xffffffff but
in a deployed system I got this output, with spaces in the MASK column:

   RTU VLAN Table Dump:

     VID    FID       MASK       DROP    PRIO    PRIO_OVERRIDE
   -----------------------------------------------------------
      0      0      0x   7ffff    NO      --         NO
     30     31      0x   7fffe    NO      --         NO
     31     31      0x   40001    NO      --         NO
    102    102      0x   7ffff    NO      --         NO

Now I get:

     VID    FID       MASK       DROP    PRIO    PRIO_OVERRIDE
   -----------------------------------------------------------
      0      0      0x0007ffff    NO      --         NO
     30     31      0x0007fffe    NO      --         NO
     31     31      0x00040001    NO      --         NO
    102    102      0x0007ffff    NO      --         NO
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>