userns: Convert the audit loginuid to be a kuid

userns: Convert the audit loginuid  to be a kuid

Always store audit loginuids in type kuid_t.

Print loginuids by converting them into uids in the appropriate user
namespace, and then printing the resulting uid.

Modify audit_get_loginuid to return a kuid_t.

Modify audit_set_loginuid to take a kuid_t.

Modify /proc/<pid>/loginuid on read to convert the loginuid into the
user namespace of the opener of the file.

Modify /proc/<pid>/loginud on write to convert the loginuid
rom the user namespace of the opener of the file.

Cc: Al Viro <[email protected]>
Cc: Eric Paris <[email protected]>
Cc: Paul Moore <[email protected]> ?
Cc: David Miller <[email protected]>
Signed-off-by: Eric W. Biederman <[email protected]>