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

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

Most forwarding tests invoke forwarding_enable() to enable the router and
forwarding_restore() to restore the original configuration. Add a helper,
adf_forwarding_enable(), which is like forwarding_enable(), 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/78b752c40069cde21c44dcf4c7b966a76a0eef2c.1758821127.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>