Handle DHCP replies with NAT44 forwarding enabled

Handle DHCP replies with NAT44 forwarding enabled

When NAT44 forwarding is disabled, if a DHCP server-to-
client packet arrives on an outside interface, it is
handled correctly by setting the next node to the next
feature on the ip4-unicast feature arc, where it can be
processed.

When NAT44 forwarding is enabled, if a DHCP server-to-
client packet arrives, it is not handled any differently
than other packets and ends up going to ip4-lookup
which results in the packet being dropped.

Move the check for DHCP server-to-client packets outside
of the block that is executed if forwarding is disabled so
DHCP replies will be processed in either case.

Change-Id: Ia795cce3fd459f3252c2c17d53bb88ceaeaafca4
Signed-off-by: Matthew Smith <mgsmith@netgate.com>