Merge branch 'cgroup/connect{4,6} programs for unprivileged ICMP ping'

YiFei Zhu says:

====================

Usually when a TCP/UDP connection is initiated, we can bind the socket
to a specific IP attached to an interface in a cgroup/connect hook.
But for pings, this is impossible, as the hook is not being called.

This series adds the invocation for cgroup/connect{4,6} programs to
unprivileged ICMP ping (i.e. ping sockets created with SOCK_DGRAM
IPPROTO_ICMP(V6) as opposed to SOCK_RAW). This also adds a test to
verify that the hooks are being called and invoking bpf_bind() from
within the hook actually binds the socket.

Patch 1 adds the invocation of the hook.
Patch 2 deduplicates write_sysctl in BPF test_progs.
Patch 3 adds the tests for this hook.

v1 -> v2:
* Added static to bindaddr_v6 in prog_tests/connect_ping.c
* Deduplicated much of the test logic in prog_tests/connect_ping.c
* Deduplicated write_sysctl() to test_progs.c

v2 -> v3:
* Renamed variable "obj" to "skel" for the BPF skeleton object in
  prog_tests/connect_ping.c

v3 -> v4:
* Fixed error path to destroy skel in prog_tests/connect_ping.c
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>