[NETLABEL]: Compilation for CONFIG_AUDIT=n case.

The audit_log_start() will expand into an empty do { } while (0)
construction and the audit_ctx becomes unused.

The solution: push current->audit_context into audit_log_start()
directly, since it is not required in any other place in the 
calling function.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>