SPARC32: Fix over-optimization by GCC near ip_fast_csum.

In some cases such as:
        iph->check = 0;
        iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);
GCC may optimize out the previous store.

Observed as a failure of NFS over udp (bad checksums on ip fragments)
when compiled with GCC 3.4.2.

Signed-off-by: Bob Breuer <breuerr@mc.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>