buffermem_pages removal (4/5)

This is the most discussion-worthy patch of the series: Change the meaning of si_meminfo->bufferram from "all pages in pagecache backed by block devices" to "all pages in pagecache".

In the header/manpage is is documented as "Memory used by buffers", but as the buffercache is gone I think the new meaning fits the intention from pre-pagecache days much better.

@@ -608,7 +608,7 @@
 	val->totalram = totalram_pages;
 	val->sharedram = 0;
 	val->freeram = nr_free_pages();
-	val->bufferram = atomic_read(&buffermem_pages);
+	val->bufferram = get_page_cache_size();
 #ifdef CONFIG_HIGHMEM
 	val->totalhigh = totalhigh_pages;
 	val->freehigh = nr_free_highpages();