adjust prefetch in free_one_pgd()

[David Mosberger <davidm@napali.hpl.hp.com>]

As per an earlier discussion on the lkml, the existing prefetch in
free_one_pgd() is somewhat broken in that it prefetches further than
PREFETCH_STRIDE bytes.  The patch below fixes that.  I'm told that
this also performs better on x86 (it makes little difference on ia64,
but it is also marginally better).

	--david

diff -Nru a/mm/memory.c b/mm/memory.c
--- a/mm/memory.c	Thu Aug  1 17:02:14 2002
+++ b/mm/memory.c	Thu Aug  1 17:02:14 2002
@@ -110,7 +110,7 @@
 	pmd = pmd_offset(dir, 0);
 	pgd_clear(dir);
 	for (j = 0; j < PTRS_PER_PMD ; j++) {
-		prefetchw(pmd+j+(PREFETCH_STRIDE/16));
+		prefetchw(pmd + j + PREFETCH_STRIDE/sizeof(*pmd));
 		free_one_pmd(tlb, pmd+j);
 	}
 	pmd_free_tlb(tlb, pmd);