selftests/bpf: Work-around EBUSY errors from hashmap update/delete

20b6cc34ea74 ("bpf: Avoid hashtab deadlock with map_locked") introduced
a possibility of getting EBUSY error on lock contention, which seems to happen
very deterministically in test_maps when running 1024 threads on low-CPU
machine. In libbpf CI case, it's a 2 CPU VM and it's hitting this 100% of the
time. Work around by retrying on EBUSY (and EAGAIN, while we are at it) after
a small sleep. sched_yield() is too agressive and fails even after 20 retries,
so I went with usleep(1) for backoff.

Also log actual error returned to make it easier to see what's going on.

Fixes: 20b6cc34ea74 ("bpf: Avoid hashtab deadlock with map_locked")
Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Song Liu <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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