map: ip4-map-t more RFC compliant

map: ip4-map-t more RFC compliant

When MTU is not set, ignore_df and mtu check
always returns true and packets are dropped.
This patch puts MTU checks after it was
compared with 0 and set to maximum if not set.
Added trace node.

If MTU is less than the total length value of
the IPv4 packet plus 20, the translator MUST
send an ICMPv4 "Fragmentation Needed" error message
to the IPv4 source address

Type: fix
Fixes: 87663cdf644fb7c94c0fec9460829b7e4e7c35ca

Signed-off-by: Vladimir Ratnikov <vratnikov@netgate.com>
Change-Id: I35b99bc2648984cdbf5b6a57ddec91c586b15bef