mm: fix comment referring to non-existent size_seqlock, change to span_seqlock

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -733,7 +733,7 @@ typedef struct pglist_data {
 	 * or node_spanned_pages stay constant.  Holding this will also
 	 * guarantee that any pfn_valid() stays that way.
 	 *
-	 * Nests above zone->lock and zone->size_seqlock.
+	 * Nests above zone->lock and zone->span_seqlock
 	 */
 	spinlock_t node_size_lock;
 #endif