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

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

Most forwarding tests invoke vrf_prepare() to set up VRF forwarding and
vrf_cleanup() to restore the original configuration. Add a helper,
adf_vrf_prepare(), which is like vrf_prepare(), but takes care of
scheduling the cleanup automatically.

Convert a number of 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/2f2000e54ae700d560a8d6128322dade3bd2207e.1758821127.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>