selftests/bpf: Wait for the struct_ops map to be freed in assoc_in_timer

st_ops_assoc_in_timer_no_uref runs .test_1, which starts a timer to
call it again 500ms later, drops every reference to the struct_ops map,
and expects the timer callback to find no struct_ops associated with
its program anymore.

The association goes away only when the last reference to the map is
dropped. syscall_prog holds one through its own association and drops
it only when it is freed, in a work item after an RCU tasks trace grace
period. With other tests running in parallel, that can take longer than
500ms, and the callback still reaches the struct_ops and gets MAP_MAGIC
instead of -1.

Have the timer callback restart the timer until the map is gone, as it
does for st_ops_assoc_in_timer_free. Calling the struct_ops through the
association until then is safe, as the map is freed only after a grace
period that follows clearing the association.

st_ops_assoc_in_timer_no_uref run 100 times with test_progs -j32 running
in the background, on a KASAN kernel with the fix, x86_64 VM with 32
vCPUs, before and after this patch:

              before                       after
  outcome     2 of 100 failed (MAP_MAGIC)  0 of 100 failed (MAP_MAGIC)

Fixes: 0e841d19263a ("selftests/bpf: Test getting associated struct_ops in timer callback")
Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261022-selftests-bpf-struct-ops-assoc-timer-v6-4-2c0f18e82582@gmail.com

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