selinux: format all invalid context as untrusted

The userspace tools expect all fields of the same name to be logged
consistently with the same encoding.  Since the invalid_context fields
contain untrusted strings in selinux_inode_setxattr()
and selinux_setprocattr(), encode all instances of this field the same
way as though they were untrusted even though
compute_sid_handle_invalid_context() and security_sid_mls_copy() are
trusted.

Please see github issue
https://github.com/linux-audit/audit-kernel/issues/57

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>