selftests/bpf: Implement BPF programs for kernel socket operations

This patch lays out a set of SYSCALL programs that can be used to invoke
the socket operation kfuncs in bpf_testmod, allowing a test program to
manipulate kernel socket operations from userspace.

Signed-off-by: Jordan Rife <jrife@google.com>
Link: https://lore.kernel.org/r/20240429214529.2644801-4-jrife@google.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>