several functions in rmap.c are searching the ptes[] array fo find the first
non-null entry.

Despite the fact tha the whole lot is in L1 cache, it is expensive,
especially on 128-byte cacheline machines.

We can encode the index of the first non-null pte entry inside the
pte_chain's `next' field and remove those searches altogether.

This reduces the rmap CPU tax by about 25% on a P4.  For a total runtime
reduction of around 5% in the bash-script intensive test which I use.