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/pnode.c b/fs/pnode.c
index 3cfd48c..58933fd 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -264,12 +264,12 @@
 		prev_src_mnt  = child;
 	}
 out:
-	br_write_lock(&vfsmount_lock);
+	lock_mount_hash();
 	while (!list_empty(&tmp_list)) {
 		child = list_first_entry(&tmp_list, struct mount, mnt_hash);
 		umount_tree(child, 0);
 	}
-	br_write_unlock(&vfsmount_lock);
+	unlock_mount_hash();
 	return ret;
 }