selftests/bpf: Fix dangling stdout seen by traffic monitor thread

Traffic monitor thread may see dangling stdout as the main thread closes
and reassigns stdout without protection. This happens when the main thread
finishes one subtest and moves to another one in the same netns_new()
scope.

The issue can be reproduced by running test_progs repeatedly with traffic
monitor enabled:

for ((i=1;i<=100;i++)); do
   ./test_progs -a flow_dissector_skb* -m '*'
done

For restoring stdout in crash_handler(), since it does not really care
about closing stdout, simlpy flush stdout and restore it to the original
one.

Then, Fix the issue by consolidating stdio_restore_cleanup() and
stdio_restore(), and protecting the use/close/assignment of stdout with
a lock. The locking in the main thread is always performed regradless of
whether traffic monitor is running or not for simplicity. It won't have
any side-effect.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20250305182057.2802606-3-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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