…ation

This commit contains a demo on how to handle tailcalls and also supports
user-space eBPF map emulation which is a part of our fake kernel model library.

We resolve tailcalls by mocking the tailcall helper function and stubbing a
callback since tailcalls are achieved by the tailcall helper function. In this
way, the callback will be called instead whenever the tailcall helper function
is encountered.

We also provide a library to support user-space map emulation. Since eBPF maps
are maintained by the kernel, we do not actually know if the map operations are
safe or not without testing in the actual kernel. Considering that eBPF maps
are actually hashmaps, we create a user-space hashmap and emulate the eBPF map
operations in the user space via callbacks.

new file:
bpf/mock/fake_maps.h: wrap up raw hashmap operations.
bpf/tests/drop_notify_test.h: include a demo test showing how to handle
tailcalls
test/bpf/drop_notify_test.c: contains main function to run test functions in drop_notify_test.h

modified:
bpf/mock/Dockerfile: add hashmap library
bpf/tests/nat_test.h: add a demo test showing how to use user-space map emulation
test/bpf/nat-test.c: add the new test in bpf/tests/nat_test.h into main
function
bpf/mock/Makefile: put all the generated files into folder "mocks" for
later cleaning
bpf/mock/mock_helpers.sh: put all the generated files into folder "mocks" for
later cleaning
test/bpf/Makefile: clean bpf/mock/mocks

Signed-off-by: Xinyuan Zhang <zhangxinyuan@google.com>