selftests/bpf: Cover test_run csum validation after bpf_skb_change_tail()

Add BPF_F_TEST_SKB_CHECKSUM_COMPLETE tests for tc programs that move the
skb out of CHECKSUM_COMPLETE with bpf_skb_change_tail():

- change_tail_trim trims the last 4 bytes of pkt_v4. They hold the
  non-zero tcp.urg_ptr, so the stale skb->csum no longer matches the
  packet.
- change_tail_grow_write appends 4 zero bytes, which on their own do
  not change the sum, and then writes a byte with BPF_F_RECOMPUTE_CSUM.
  That write can no longer update skb->csum.

Both return -EBADMSG without the preceding test_run fix and pass with
it.

Also add store_no_recompute as a negative test. It writes a byte
without BPF_F_RECOMPUTE_CSUM, so the skb stays CHECKSUM_COMPLETE with a
stale skb->csum, and test_run must keep returning -EBADMSG. This checks
that the validation still runs for CHECKSUM_COMPLETE skbs. The passing
case, a CHECKSUM_COMPLETE skb whose checksum is updated correctly, is
already covered by test_skb_pkt_end.

Signed-off-by: Maxim Skokov <skokovmaksimevg@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261009174755.22026-3-skokovmaksimevg@gmail.com

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