selftests: forwarding: gre_multipath: Enable IPv4 forwarding

The test did not enable IPv4 forwarding during its setup phase, which
causes the test to fail on machines where IPv4 forwarding is disabled.

Fixes: 54818c4c4b93 ("selftests: forwarding: Test multipath tunneling")
Signed-off-by: Ido Schimmel <[e-mail]>
Reported-by: Stephen Suryaputra <[e-mail]>
Tested-by: Stephen Suryaputra <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/forwarding/gre_multipath.sh

···
 	sw1_create
 	sw2_create
 	h2_create
+
+	forwarding_enable
 }
 
 cleanup()
 {
 	pre_cleanup
+
+	forwarding_restore
 
 	h2_destroy
 	sw2_destroy