selftests: Fix failing VXLAN VNI filtering test

iproute2 does not recognize the "group6" and "remote6" keywords. Fix by
using "group" and "remote" instead.

Before:

 # ./test_vxlan_vnifiltering.sh
 [...]
 Tests passed:  25
 Tests failed:   2

After:

 # ./test_vxlan_vnifiltering.sh
 [...]
 Tests passed:  27
 Tests failed:   0

Fixes: 3edf5f66

 ("selftests: add new tests for vxlan vnifiltering")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://lore.kernel.org/r/20230207141819.256689-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>