bpf/selftests: Fix namespace mount setup in tc_redirect

The tc_redirect umounts /sys in the new namespace, which can be
mounted as shared and cause global umount. The lazy umount also
takes down mounted trees under /sys like debugfs, which won't be
available after sysfs mounts again and could cause fails in other
tests.

  # cat /proc/self/mountinfo | grep debugfs
  34 23 0:7 / /sys/kernel/debug rw,nosuid,nodev,noexec,relatime shared:14 - debugfs debugfs rw
  # cat /proc/self/mountinfo | grep sysfs
  23 86 0:22 / /sys rw,nosuid,nodev,noexec,relatime shared:2 - sysfs sysfs rw
  # mount | grep debugfs
  debugfs on /sys/kernel/debug type debugfs (rw,nosuid,nodev,noexec,relatime)

  # ./test_progs -t tc_redirect
  #164 tc_redirect:OK
  Summary: 1/4 PASSED, 0 SKIPPED, 0 FAILED

  # mount | grep debugfs
  # cat /proc/self/mountinfo | grep debugfs
  # cat /proc/self/mountinfo | grep sysfs
  25 86 0:22 / /sys rw,relatime shared:2 - sysfs sysfs rw

Making the sysfs private under the new namespace so the umount won't
trigger the global sysfs umount.

Reported-by: Hangbin Liu <haliu@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jussi Maki <joamaki@gmail.com>
Link: https://lore.kernel.org/bpf/20220104121030.138216-1-jolsa@kernel.org