new helpers: lock_mount_hash/unlock_mount_hash

aka br_write_{lock,unlock} of vfsmount_lock.  Inlines in fs/mount.h,
vfsmount_lock extern moved over there as well.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 9f4935b..09c1132 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -26,6 +26,7 @@
 #include <linux/blkdev.h>
 #include <linux/backing-dev.h>
 #include <linux/tracepoint.h>
+#include <linux/device.h>
 #include "internal.h"
 
 /*