In rmnet_map_ipv4_dl_csum_trailer(), remove the "csum_temp" and
"addend" local variables, and simplify a few lines of code.

Remove the "csum_temp", "csum_value", "ip6_hdr_csum", and "addend"
local variables in rmnet_map_ipv6_dl_csum_trailer(), and simplify a
few lines of code there as well.
Signed-off-by: NAlex Elder <elder@linaro.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>