kselftest/arm64: Fix SVE write data generation for SME only systems

kselftest/arm64: Fix SVE write data generation for SME only systems

fp-ptrace does not handle SME only systems correctly when generating data,
on SME only systems scenarios where we are not in streaming mode will not
have an expected vector length. This leads to attempts to do memcpy()s of
zero byte arrays which can crash, fix this by skipping generation of SVE
data for cases where we do not expect to have an active vector length.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250718-arm64-fp-ptrace-sme-only-v1-2-3b96dd19a503@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>