[PATCH] Avoid recursion in the page allocator

The PF_MEMALLOC handling got broken somewhere, and it is now possible
for a PF_MEMALLOC process to reenter page reclaim.

Change it to fail the allocation if we're PF_MEMALLOC and there are
zero pages free.

mm/page_alloc.c

@@ -506,6 +506,7 @@ __alloc_pages(unsigned int gfp_mask, unsigned int order,
 			if (page)
 				return page;
 		}
+		goto nopage;
 	}
 
 	/* Atomic allocations - we can't balance anything */