The value passed as an argument to rmnet_map_ipv4_ul_csum_header()
is always an IPv4 header.  Rather than using a local variable, just
have the type of the argument reflect the proper type.

In rmnet_map_ipv6_ul_csum_header() things are defined a little
differently, but make the same basic change there.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>