[NET]: Make illegal_highdma more anal

Rather than having illegal_highdma as a macro when HIGHMEM is off, we
can turn it into an inline function that returns zero.  This will catch
callers that give it bad arguments.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>