UBUNTU: SAUCE: (namespace) fuse: Restrict allow_other to the superblock's namespace...

UBUNTU: SAUCE: (namespace) fuse: Restrict allow_other to the superblock's namespace or a descendant

Unprivileged users are normally restricted from mounting with the
allow_other option by system policy, but this could be bypassed
for a mount done with user namespace root permissions. In such
cases allow_other should not allow users outside the userns
to access the mount as doing so would give the unprivileged user
the ability to manipulate processes it would otherwise be unable
to manipulate. Restrict allow_other to apply to users in the same
userns used at mount or a descendant of that namespace. Also
export current_in_userns() for use by fuse when built as a
module.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>