userns: Add user namespace support to IMA

Use kuid's in the IMA rules.

When reporting the current uid in audit logs use from_kuid
to get a usable value.

Cc: Mimi Zohar <zohar@us.ibm.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>