Re-enable socket policy test by invalidating the dst cache.

In current kernels, applying a socket policy does not
invalidate the socket dst cache entry, so future sends on the
socket cause packets to be sent in the clear. Instead of
skipping the test on these kernels, forcibly clear socket
dst cache to make the test pass.

This is likely not feasible for production code because we
generally do not know the netid that a socket is bound on when
we apply a policy, but at least we can continue to run the test
without fear of bitrot or regressions.

Bug: 63589916
Test: xfrm_test.py passes on 4.9 device kernel
Change-Id: Ib8df6995b2a253f6260d426ee7cb3273836a2598