netfilter: nf_nat: add support for persistent mappings

The removal of the SAME target accidentally removed one feature that is
not available from the normal NAT targets so far, having multi-range
mappings that use the same mapping for each connection from a single
client. The current behaviour is to choose the address from the range
based on source and destination IP, which breaks when communicating
with sites having multiple addresses that require all connections to
originate from the same IP address.

Introduce a IP_NAT_RANGE_PERSISTENT option that controls whether the
destination address is taken into account for selecting addresses.

http://bugzilla.kernel.org/show_bug.cgi?id=12954



Signed-off-by: Patrick McHardy <kaber@trash.net>