selftests: net: lib: Add several autodefer helpers

Add ip_link_set_addr(), ip_link_set_up(), ip_addr_add() and ip_route_add()
to the suite of helpers that automatically schedule a corresponding
cleanup.

When setting a new MAC, one needs to remember the old address first. Move
mac_get() from forwarding/ to that end.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/add6bcbe30828fd01363266df20c338cf13aaf25.1733412063.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>