nat: deny adding intf addr if static-mapping-only

nat: deny adding intf addr if static-mapping-only

If static-mapping-only is enabled, NAT pool cannot be configured, only
static mappings. There're two ways to add addresses to the NAT pool:
by address range, or by first found address from an interface.
NAT44_ADD_DEL_ADDRESS_RANGE already tests if dynamic mappings are
available but NAT44_ADD_DEL_INTERFACE_ADDR doesn't. If
static-mapping-only is enabled, adding addresses by range is rejected
but by interface not.

With this change, if static-mapping-only is enabled, do not allow to
add addresses to the NAT pool both ways.

Type: fix
Signed-off-by: Alexander Chernavin <achernavin@netgate.com>
Change-Id: Ifc055ea9a71a5e579388833a2990aef21bf7ed29