selftests: forwarding: lib: Add an autodefer variant of simple_if_init()

selftests: forwarding: lib: Add an autodefer variant of simple_if_init()

Most forwarding tests invoke simple_if_init() to set up a VRF-based "host"
and simple_if_fini() to tear it down again. Add a helper,
adf_simple_if_init(), which is like simple_if_fini(), but takes care of
scheduling the cleanup automatically.

Convert the tests that currently use defer to schedule the cleanup.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/6b9ee1a7946a36fd32a47fdb1aa9325198ffc695.1758821127.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>