Verify that total device stats don't decrease after it has been turned down.
Also make sure the device doesn't crash when we access per-queue stats
when it's down (in case it tries to access some pointers that are NULL).

  KTAP version 1
  1..5
  ok 1 stats.check_pause
  ok 2 stats.check_fec
  ok 3 stats.pkt_byte_sum
  ok 4 stats.qstat_by_ifindex
  ok 5 stats.check_down
  # Totals: pass:5 fail:0 xfail:0 xpass:0 skip:0 error:0

v3:
- use errno.EOPNOTSUPP (Petr)
- move qstat[0] under try (Petr)

v2:
- KTAP output formatting (Jakub)
- defer instead of try/finally (Jakub)
- disappearing stats is an error (Jakub)
- ksft_ge instead of open coding (Jakub)
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20240802000309.2368-1-sdf@fomichev.meSigned-off-by: Jakub Kicinski <kuba@kernel.org>