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.

This is essentially a cherry-pick of change 30284 but the
automatic cherry-picking did not work because of some filename
changes.

Type: fix

Signed-off-by: Elias Rudberg <elias.rudberg@bahnhof.net>
Change-Id: I21a59ae7423f40abeff9fc0411330da58b3011f0