kselftest/arm64: Correct buffer allocation for SVE Z registers

kselftest/arm64: Correct buffer allocation for SVE Z registers

The buffer used for verifying SVE Z registers allocated enough space for
16 maximally sized registers rather than 32 due to using the macro for the
number of P registers. In practice this didn't matter since for historical
reasons the maximum VQ defined in the ABI is greater the architectural
maximum so we will always allocate more space than is needed even with
emulated platforms implementing the architectural maximum. Still, we should
use the right define.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220829162502.886816-2-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>