[NETFILTER]: x_tables: small check_entry & module_refcount cleanup

While standard_target has target->me == NULL, module_put() should be
called for it as for others, because there were try_module_get() before.

Signed-off-by: Dmitry Mishin <dim@openvz.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>