selftests: rtnetlink: remove netdevsim device after ipsec offload test

On systems where netdevsim is built-in or loaded before the test
starts, kci_test_ipsec_offload doesn't remove the netdevsim device it
created during the test.

Fixes: e05b2d14 ("netdevsim: move netdev creation/destruction to dev probe")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/e1cb94f4f82f4eca4a444feec4488a1323396357.1687466906.git.sd@queasysnail.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/rtnetlink.sh

@@ -860,6 +860,7 @@ EOF
 	fi
 
 	# clean up any leftovers
+	echo 0 > /sys/bus/netdevsim/del_device
 	$probed && rmmod netdevsim
 
 	if [ $ret -ne 0 ]; then