mm: remove useless rcu lock-unlock from mapping_tagged()

radix_tree_tagged() is lockless - it reads from a member of the raid-tree
root node.  It does not require any protection.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -1405,10 +1405,6 @@ EXPORT_SYMBOL(test_set_page_writeback);
  */
 int mapping_tagged(struct address_space *mapping, int tag)
 {
-	int ret;
-	rcu_read_lock();
-	ret = radix_tree_tagged(&mapping->page_tree, tag);
-	rcu_read_unlock();
-	return ret;
+	return radix_tree_tagged(&mapping->page_tree, tag);
 }
 EXPORT_SYMBOL(mapping_tagged);