netfilter: nf_nat: no IP_NAT_RANGE_MAP_IPS flags when alloc_null_binding()

When alloc_null_binding(), no IP_NAT_RNAGE_MAP_IPS in flags means no IP address
translation is needed. It isn't necessary to specify the address explicitly.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>