selftests: forwarding: lib: Extract trap_{, un}install()

A mirror-to-vlan test that's coming next needs to install the trap
unconditionally. Therefore extract from slow_path_trap_{,un}install()
a more generic functions trap_install() and trap_uninstall(), and covert
the former two to conditional wrappers around these.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>