In rmnet_map_ipv4_dl_csum_trailer(), if the sum of the trailer
checksum and the pseudo checksum is non-zero, checksum validation
has failed.  We can return an error as soon as we know that.

We can do the same thing in rmnet_map_ipv6_dl_csum_trailer().

Add some comments that explain where we're headed.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>