selftests/bpf: Fix error failure of case test_xdp_adjust_tail_grow

test_xdp_adjust_tail_grow failed with ipv6:
  test_xdp_adjust_tail_grow:FAIL:ipv6 unexpected error: -28 (errno 28)

The reason is that this test case tests ipv4 before ipv6, and when ipv4
test finished, topts.data_size_out was set to 54, which is smaller than the
ipv6 output data size 114, so ipv6 test fails with NOSPC error.

Fix it by reset topts.data_size_out to sizeof(buf) before testing ipv6.

Fixes: 04fcb5f9

 ("selftests/bpf: Migrate from bpf_prog_test_run")
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/bpf/20221011120108.782373-6-xukuohai@huaweicloud.com