nat: prevent creation when-resolved static mappings on errors

nat: prevent creation when-resolved static mappings on errors

When you create two identical NAT44 static mappings using interface
name as external address and only local or ext port is different,
VALUE_EXIST will be raised but when-resolved static mapping will
remain.

vpp# nat44 add static mapping tcp local 10.128.0.129 443 external GigabitEthernet0/8/0 8443
vpp# nat44 add static mapping tcp local 10.128.0.129 80 external GigabitEthernet0/8/0 8443
nat44 add static mapping: Mapping already exist.
vpp# show nat44 static mappings
NAT44 static mappings:
 tcp local 10.128.0.129:443 external 2.2.2.2:8443 vrf 0
 tcp local 10.128.0.129:443 external GigabitEthernet0/8/0:8443 vrf -1
 tcp local 10.128.0.129:80 external GigabitEthernet0/8/0:8443 vrf -1

With this commit, when-resolved static mapping is not created if the
translation only differs in local or ext port.

Change-Id: Ifc960b9dc1371caa2a8d3206a80a0ffd10d293e4
Signed-off-by: Alexander Chernavin <achernavin@netgate.com>