The problem with that implementation is that it has to fork lots of
subshells.  I was thinking more along the lines of:

mkdisk_arith() {
  mknod $1 b $2 $(($3 * 16 + $4))
}
mkdisk_expr() {
  mknod $1 b $2 `expr $3 * 16 + $4`
}

...
sd*)
...
  mkdisk_$foo sd${unit}c $maj $unit 2
...