nat: avoid hairpinning infinite loop problem

nat: avoid hairpinning infinite loop problem

Fix in nat44 hairpinning code to check if anything was actually
changed in the snat_hairpinning() routine, and return 0 if nothing
changed. This helps avoid an infinite loop repeating the three
nodes nat44-hairpinning-->ip4-lookup-->ip4-local in case there
was no change. Also add a corresponding test case.

Type: fix

Signed-off-by: Elias Rudberg <elias.rudberg@bahnhof.net>
Change-Id: I95f48476bd002ac4c6789afe504681f1963e5d38
Signed-off-by: Elias Rudberg <elias.rudberg@bahnhof.net>