dmapools: protect page_list walk in show_pools()

show_pools() walks the page_list of a pool w/o protection against the list
modifications in alloc/free. Take pool->lock to avoid stomping into
nirvana.

Signed-off-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Matthew Wilcox <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/dmapool.c

···
 		unsigned pages = 0;
 		unsigned blocks = 0;
 
+		spin_lock_irq(&pool->lock);
 		list_for_each_entry(page, &pool->page_list, page_list) {
 			pages++;
 			blocks += page->in_use;
 		}
+		spin_unlock_irq(&pool->lock);
 
 		/* per-pool info, no real statistics yet */
 		temp = scnprintf(next, size, "%-16s %4u %4Zu %4Zu %2u\n",