sys/dev/disk/dm: Fix sanity checks for striped target [2/4]

sys/dev/disk/dm: Fix sanity checks for striped target [2/4]

Target specific args for striped is redundant in the sense
that it has an arg for # of stripes other than the args for
actual stripe devices. These two must meet below.

((argc - 2) / 2) == # of stripe devices.

e.g.
--table '0 100 stripe1 3 10 /dev/da3 0 /dev/da4 0 /dev/da5 0'
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 8 target args
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 6 device args
                       ^ # of stripe devices
  ((8 - 2) / 2) = 6/2 = 3
      -> 3 stripe devices (/dev/da[345])