Merge branch 'bpf-test_run-fix-false-ebadmsg-with-bpf_f_test_skb_checksum_complete'

Maxim Skokov says:

====================
bpf: test_run: Fix false -EBADMSG with BPF_F_TEST_SKB_CHECKSUM_COMPLETE

BPF_PROG_TEST_RUN with BPF_F_TEST_SKB_CHECKSUM_COMPLETE returns -EBADMSG
for correct tc programs that call bpf_skb_change_tail(). The helper
moves the skb to CHECKSUM_NONE and leaves skb->csum stale, but the
post-run check added by commit a3cfe84cca28 ("bpf: Add CHECKSUM_COMPLETE
to bpf test progs") still compares against it.

Patch 1 only does the check while the skb is still CHECKSUM_COMPLETE.
Patch 2 adds selftests for a trim, a grow followed by a write, and a
negative case that keeps the skb CHECKSUM_COMPLETE with a stale
skb->csum.

Tested in a VM on bpf, with two kernels that differ only by patch 1
(subtest results):

                           without patch 1    with patch 1
  change_tail_trim         FAIL (-EBADMSG)    OK
  change_tail_grow_write   FAIL (-EBADMSG)    OK
  store_no_recompute       OK                 OK
  test_skb_pkt_end         OK                 OK

Changes in v2:
- Put the opening /* of multi-line comments on its own line (bpf-ci,
  Sashiko).

v1: https://lore.kernel.org/bpf/20261007181249.352831-1-skokovmaksimevg@gmail.com/
====================

Link: https://patch.msgid.link/20261009174755.22026-1-skokovmaksimevg@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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