selftests/powerpc/spectre_v2: Return skip code when miss_percent is high

selftests/powerpc/spectre_v2: Return skip code when miss_percent is high

[ Upstream commit 3c42e9542050d49610077e083c7c3f5fd5e26820 ]

A mis-match between reported and actual mitigation is not restricted to the
Vulnerable case. The guest might also report the mitigation as "Software
count cache flush" and the host will still mitigate with branch cache
disabled.

So, instead of skipping depending on the detected mitigation, simply skip
whenever the detected miss_percent is the expected one for a fully
mitigated system, that is, above 95%.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211207130557.40566-1-cascardo@canonical.com
Signed-off-by: Sasha Levin <sashal@kernel.org>