#ifdef very expensive debug check in page fault path

This patch puts #ifdef CONFIG_DEBUG_VM around a check in vm_normal_page
that verifies that a pfn is valid.  This patch increases performance of the
page fault microbenchmark in lmbench by 13% and overall dbench performance
by 7% on s390x.  pfn_valid() is an expensive operation on s390 that needs a
high double digit amount of CPU cycles.  Nick Piggin suggested that
pfn_valid() involves an array lookup on systems with sparsemem, and
therefore is an expensive operation there too.

The check looks like a clear debug thing to me, it should never trigger on
regular kernels.  And if a pte is created for an invalid pfn, we'll find
out once the memory gets accessed later on anyway.  Please consider
inclusion of this patch into mm.

Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>