capabilities: Don't allow writing ambiguous v3 file capabilities

The v3 file capabilities have a uid field that records the filesystem
uid of the root user of the user namespace the file capabilities are
valid in.

When someone is silly enough to have the same underlying uid as the
root uid of multiple nested containers a v3 filesystem capability can
be ambiguous.

In the spirit of don't do that then, forbid writing a v3 filesystem
capability if it is ambiguous.

Fixes: 8db6c34f

 ("Introduce v3 namespaced file capabilities")
Reviewed-by: Andrew G. Morgan <morgan@kernel.org>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>