kselftest/arm64: Expand the set of ZA writes fp-ptrace does

Currently our test for implementable ZA writes is written in a bit of a
convoluted fashion which excludes all changes where we clear SVCR.SM even
though we can actually support that since changing the vector length resets
SVCR. Make the logic more direct, enabling us to actually run these cases.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241112-arm64-fp-ptrace-fpmr-v2-2-250b57c61254@kernel.org


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