kselftest/arm64: Fix syscall-abi for systems without 128 bit SME

SME does not mandate any specific VL so we may not have 128 bit SME but
the algorithm used for enumerating VLs assumes that we will. Add the
required check to ensure that the algorithm terminates.

Fixes: 43e3f855 ("kselftest/arm64: Add SME support to syscall ABI test")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221223-arm64-syscall-abi-sme-only-v1-1-4fabfbd62087@kernel.org


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