kselftest/arm64: Tighten up validation of ZA signal context

Currently we accept any size for the ZA signal context that the shared
code will accept which means we don't verify that any data is present.
Since we have enabled ZA we know that there must be data so strengthen
the check to only accept a signal frame with data, and while we're at it
since we enabled ZA but did not set any data we know that ZA must contain
zeros, confirm that.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220829155728.854947-2-broonie@kernel.org


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