Verify VTI Modification using RTM_NEWLINK

This test checks that an existing VTI(6) can be
updated using the RTM_NEWLINK without the EXCL
flag.

Bug: 72532478
Test: aosp/kernel/tests/net/test/run_net_test.sh
Test: kernel_net_tests (on wahoo/walleye)
Merged-In: Iecb7bb7f070dfbdd098dbb773d18ca21f23a42da
Change-Id: Iecb7bb7f070dfbdd098dbb773d18ca21f23a42da
(cherry picked from commit 3f132dc2)