vfs: Don't allow a user namespace root to make device nodes

Safely making device nodes in a container is solvable but simply
having the capability in a user namespace is not sufficient to make
this work.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

diff --cc fs/namei.c
Simple merge