mm: fix invalid use of pfn_valid_within in test_pages_in_a_zone

mm: fix invalid use of pfn_valid_within in test_pages_in_a_zone

Offlining memory by 'echo 0 > /sys/devices/system/memory/memory#/online'
or reading valid_zones 'cat
/sys/devices/system/memory/memory#/valid_zones' causes BUG: unable to
handle kernel paging request due to invalid use of pfn_valid_within.  This
is due to a bug in test_pages_in_a_zone.

In order to use pfn_valid_within within a MAX_ORDER_NR_PAGES block of
pages, a valid pfn within the block must first be found.  There only needs
to be one valid pfn found in test_pages_in_a_zone in the first place.  So
the fix is to replace pfn_valid_within with pfn_valid such that the first
valid pfn within the pageblock is found (if it exists).  This works
independently of CONFIG_HOLES_IN_ZONE.

Signed-off-by: James Custer <jcuster@sgi.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Russ Anderson <rja@sgi.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>