selftests/bpf: Test direct packet access on non-linear skbs

selftests/bpf: Test direct packet access on non-linear skbs

This patch adds new selftests in the direct packet access suite, to
cover the non-linear case. The first six tests cover the behavior of
the bounds check with a non-linear skb. The last test adds a call to
bpf_skb_pull_data() to be able to access the packet.

Note that the size of the linear area includes the L2 header, but for
some program types like cgroup_skb, ctx->data points to the L3 header.
Therefore, a linear area of 22 bytes will have only 8 bytes accessible
to the BPF program (22 - ETH_HLEN). For that reason, the cgroup_skb test
cases access the packet at an offset of 8 bytes.

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/ceedbfd719e58f0d49dcceb8592f5e6bd38ce5fe.1760037899.git.paul.chaignon@gmail.com