arm64/fpsimd: signal: Mandate SVE payload for streaming-mode state

Non-streaming SVE state may be preserved without an SVE payload, in
which case the SVE context only has a header with VL==0, and all state
can be restored from the FPSIMD context. Streaming SVE state is always
preserved with an SVE payload, where the SVE context header has VL!=0,
and the SVE_SIG_FLAG_SM flag is set.

The kernel never preserves an SVE context where SVE_SIG_FLAG_SM is set
without an SVE payload. However, restore_sve_fpsimd_context() doesn't
forbid restoring such a context, and will handle this case by clearing
PSTATE.SM and restoring the FPSIMD context into non-streaming mode,
which isn't consistent with the SVE_SIG_FLAG_SM flag.

Forbid this case, and mandate an SVE payload when the SVE_SIG_FLAG_SM
flag is set. This avoids an awkward ABI quirk and reduces the risk that
later rework to this code permits configuring a task with PSTATE.SM==1
and fp_type==FP_STATE_FPSIMD.

I've marked this as a fix given that we never intended to support this
case, and we don't want anyone to start relying upon the old behaviour
once we re-enable SME.

Fixes: 85ed24dad290 ("arm64/sme: Implement streaming SVE signal handling")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-4-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

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