selftests/bpf: Test invalid narrower ctx load

This patch adds selftests to cover invalid narrower loads on the
context. These used to cause kernel warnings before the previous patch.
To trigger the warning, the load had to be aligned, to read an affected
context field (ex., skb->sk), and not starting at the beginning of the
field.

The nine new cases all fail without the previous patch.

Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/44cd83ea9c6868079943f0a436c6efa850528cc1.1753194596.git.paul.chaignon@gmail.com