bpf: Craft non-linear skbs in BPF_PROG_TEST_RUN

This patch adds support for crafting non-linear skbs in BPF test runs
for tc programs. The size of the linear area is given by ctx->data_end,
with a minimum of ETH_HLEN always pulled in the linear area. If ctx or
ctx->data_end are null, a linear skb is used.

This is particularly useful to test support for non-linear skbs in large
codebases such as Cilium. We've had multiple bugs in the past few years
where we were missing calls to bpf_skb_pull_data(). This support in
BPF_PROG_TEST_RUN would allow us to automatically cover this case in our
BPF tests.

LWT program types are currently excluded in this patch. Allowing
non-linear skbs for these programs would require a bit more care because
they are able to call helpers (ex., bpf_clone_redirect, bpf_redirect)
that themselves call eth_type_trans(). eth_type_trans() assumes there
are at least ETH_HLEN bytes in the linear area. That may not be true
for LWT programs as we already pulled the L2 header via the
eth_type_trans() call in bpf_prog_test_run_skb().

In addition to the selftests introduced later in the series, this patch
was tested by enabling non-linear skbs for all tc selftests programs
and checking test failures were expected.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Tested-by: syzbot@syzkaller.appspotmail.com
Link: https://patch.msgid.link/5694d4d1af31bddf974afcb1bbb1e28b8998dcd0.1760037899.git.paul.chaignon@gmail.com

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