selftsets/bpf: Retry map update on helper_fill_hashmap()

helper_fill_hashmap() is used also on parallel and stress map tests.
Those are consistently failing with ENOMEM on kernels built with
PREEMPT_RT if preallocation is disabled. The failure is transient and
only called by the memory cache refill running in a preemptible
irq_work, which can easily stall in case of contention.

Use a retriable update in those cases to handle transient ENOMEM and
make the test more stable also on PREEMPT_RT.
Also fix the sign of the value printed in case of error (strerror()
expects a positive errno while updates return it negative).

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260611150704.95133-1-gmonaco@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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