net: fix sparse error in csum_replace4()

make C=2 CF=-D__CHECK_ENDIAN__ net/ipv4/netfilter/nf_nat_l3proto_ipv4.o
  CHECK   net/ipv4/netfilter/nf_nat_l3proto_ipv4.c
include/net/checksum.h:125:64: warning: incorrect type in argument 2 (different base types)
include/net/checksum.h:125:64:    expected restricted __wsum [usertype] addend
include/net/checksum.h:125:64:    got restricted __be32 [usertype] from
include/net/checksum.h:125:71: warning: incorrect type in argument 2 (different base types)
include/net/checksum.h:125:71:    expected restricted __wsum [usertype] addend
include/net/checksum.h:125:71:    got restricted __be32 [usertype] to
include/net/checksum.h:125:64: warning: incorrect type in argument 2 (different base types)
include/net/checksum.h:125:64:    expected restricted __wsum [usertype] addend
include/net/checksum.h:125:64:    got restricted __be32 [usertype] from
include/net/checksum.h:125:71: warning: incorrect type in argument 2 (different base types)
include/net/checksum.h:125:71:    expected restricted __wsum [usertype] addend
include/net/checksum.h:125:71:    got restricted __be32 [usertype] to

Fixes: 4565af0d ("net: optimise csum_replace4()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>