Update ND_BYTES_BETWEEN() macro for better accuracy

Update ND_BYTES_BETWEEN() macro for better accuracy

Update the macro that computes how many bytes are present, starting
at the first argument and running up to (but not including) the second
argument, and returns that as a u_int (cutting it to 32 bits on LP64
and LLP64 platforms).

This reverses, for reasons of readability, the order of the arguments
which was based on old SMB's PTR_DIFF().

With this change the number of bytes "between" given by the macro is 0
when the first argument is greater than or equal to the second argument.

Update ND_BYTES_AVAILABLE_AFTER() accordingly.

This is a follow-up to f9c2c905b118b69a0b102549c1b25cca871947b5.