NetLabel: honor the audit_enabled flag

The audit_enabled flag is used to signal when syscall auditing is to be
performed.  While NetLabel uses a Netlink interface instead of syscalls, it is
reasonable to consider the NetLabel Netlink interface as a form of syscall so
pay attention to the audit_enabled flag when generating audit messages in
NetLabel.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>