selftests: drv-net: test drivers sleeping in ndo_get_stats64

selftests: drv-net: test drivers sleeping in ndo_get_stats64

Most of our tests use rtnetlink to read device stats, so they
don't expose the drivers much to paths in which device stats
are read under RCU. Add tests which hammer profcs reads to
make sure drivers:
 - don't sleep while reporting stats,
 - can handle parallel reads,
 - can handle device going down while reading.

Set ifname on the env class in NetDrvEnv, we already do that
in NetDrvEpEnv.

  KTAP version 1
  1..7
  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
  ok 6 stats.procfs_hammer
  # completed up/down cycles: 6
  ok 7 stats.procfs_downup_hammer
  # Totals: pass:7 fail:0 xfail:0 xpass:0 skip:0 error:0

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250107022932.2087744-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>