net: core: Use csum_replace_by_diff() and csum_sub() instead of opencoding

Open coded calculation can be avoided and replaced by the
equivalent csum_replace_by_diff() and csum_sub().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>