kselftest/arm64: Validate vector lengths are set in sve-probe-vls

Currently sve-probe-vls does not verify that the vector lengths reported
by the prctl() interface are actually what is reported by the architecture,
use the rdvl_sve() helper to validate this.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Link: https://lore.kernel.org/r/20210803140450.46624-3-broonie@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>