Patch from Hugh Dickins <hugh@veritas.com>

On some architectures (cachetlb.txt gives HyperSparc as an example)
it is essential to flush_cache_page while pte is still valid: the
rmap VM diverged from the base 2.4 VM before that fix was made,
so this error has crept back into 2.5.

Patch below applies to 2.5.47 or 2.5.47-mm1 - needs more work over
shared pagetables, but they've silently fallen out of 2.5.47-mm1:
oversight?  I'll send Alan the equivalent for 2.4-ac shortly.

(I wonder, what happens if userspace now modifies the page
after the flush_cache_page, before the pte is invalidated?)