mm: use zone_end_pfn() instead of zone_start_pfn+spanned_pages

Use "zone_end_pfn()" instead of "zone->zone_start_pfn + zone->spanned_pages".
Simplify the code, no functional change.

[akpm@linux-foundation.org: fix build]
Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Cc: Cody P Schafer <cody@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>