For PA-RISC, we need find_vma_prev to return `prev', even if vma is NULL.
Our stack is at the top of memory, growing upwards, so when we page fault
we need to see prev.  For added bonus points, the code becomes simpler,
less indented, shorter and (for me, anyway) easier to understand.  The
code is well-tested, even on x86.  For PA and ia64 this code is called in
the page fault handler path so it is exercised frequently.