selftests/bpf: Fix error compiling tc_redirect.c with musl libc

selftests/bpf: Fix error compiling tc_redirect.c with musl libc

Linux 5.1 implemented 64-bit time types and related syscalls to address the
Y2038 problem generally across archs. Userspace handling of Y2038 varies
with the libc however. While musl libc uses 64-bit time across all 32-bit
and 64-bit platforms, GNU glibc uses 64-bit time on 64-bit platforms but
defaults to 32-bit time on 32-bit platforms unless they "opt-in" to 64-bit
time or explicitly use 64-bit syscalls and time structures.

One specific area is the standard setsockopt() call, SO_TIMESTAMPNS option
used for timestamping, and the related output 'struct timespec'. GNU glibc
defaults as above, also exposing the SO_TIMESTAMPNS_NEW flag to explicitly
use a 64-bit call and 'struct __kernel_timespec'. Since these are not
exposed or needed with musl libc, their use in tc_redirect.c leads to
compile errors building for mips64el/musl:

  tc_redirect.c: In function 'rcv_tstamp':
  tc_redirect.c:425:32: error: 'SO_TIMESTAMPNS_NEW' undeclared (first use in this function); did you mean 'SO_TIMESTAMPNS'?
    425 |             cmsg->cmsg_type == SO_TIMESTAMPNS_NEW)
        |                                ^~~~~~~~~~~~~~~~~~
        |                                SO_TIMESTAMPNS
  tc_redirect.c:425:32: note: each undeclared identifier is reported only once for each function it appears in
  tc_redirect.c: In function 'test_inet_dtime':
  tc_redirect.c:491:49: error: 'SO_TIMESTAMPNS_NEW' undeclared (first use in this function); did you mean 'SO_TIMESTAMPNS'?
    491 |         err = setsockopt(listen_fd, SOL_SOCKET, SO_TIMESTAMPNS_NEW,
        |                                                 ^~~~~~~~~~~~~~~~~~
        |                                                 SO_TIMESTAMPNS

However, using SO_TIMESTAMPNS_NEW isn't strictly needed, nor is Y2038 being
explicitly tested. The timestamp checks in tc_redirect.c are simple: the
packet receive timestamp is non-zero and processed/handled in less than 5
seconds.

Switch to using the standard setsockopt() call and SO_TIMESTAMPNS option to
ensure compatibility across glibc and musl libc. In the worst-case, there
is a 5-second window 14 years from now where tc_redirect tests may fail on
32-bit systems. However, we should reasonably expect glibc to adopt a
64-bit mandate rather than the current "opt-in" policy before the Y2038
roll-over.

Fixes: ce6f6cffaeaa ("selftests/bpf: Wait for the netstamp_needed_key static key to be turned on")
Fixes: c803475fd8dd ("bpf: selftests: test skb->tstamp in redirect_neigh")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/031d656c058b4e55ceae56ef49c4e1729b5090f3.1722244708.git.tony.ambardar@gmail.com