userns: Allow unprivilged mounts of proc and sysfs

- The context in which proc and sysfs are mounted have no
  effect on the the uid/gid of their files so no conversion is
  needed except allowing the mount.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index 71eb7e2..db940a9 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -149,6 +149,7 @@
 	.name		= "sysfs",
 	.mount		= sysfs_mount,
 	.kill_sb	= sysfs_kill_sb,
+	.fs_flags	= FS_USERNS_MOUNT,
 };
 
 int __init sysfs_init(void)