The hammer branch based gcc 3.3 in SuSE 9.0 has a more aggressive
optimizer. ip_send_check has this code:

	iph->check = 0;
	iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);

The new gcc optimizes the first store away because it doesn't know
that ip_fast_csum reads its input memory. This leads to occassionally
packets with wrong IP header checksum getting sent; this happens especially
with NFS. Fixing it in the constraints would have been ugly and probably
not future proof, so this patch just adds a memory clobber to ip_fast_csum.

For some reason the issue only hits in 2.6, we haven't seen it in 2.4.
Problem occurs on both i386 and x86-64.

Credit goes to Olaf Kirch for tracking this down.