net_sched: convert tcindex to call tcf_exts_destroy from rcu callback

Adjust destroy path of cls_tcindex to call tcf_exts_destroy() after
rcu grace period.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>