arm64/signal: Always accept SVE signal frames on SME only systems

Currently we reject an attempt to restore a SVE signal frame on a system
with SME but not SVE supported. This means that it is not possible to
disable streaming mode via signal return as this is configured via the
flags in the SVE signal context. Instead accept the signal frame, we will
require it to have a vector length of 0 specified and no payload since the
task will have no SVE vector length configured.

Fixes: 85ed24da ("arm64/sme: Implement streaming SVE signal handling")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221223-arm64-fix-sme-only-v1-2-938d663f69e5@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/signal.c

@@ -281,7 +281,12 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
 
 		vl = task_get_sme_vl(current);
 	} else {
-		if (!system_supports_sve())
+		/*
+		 * A SME only system use SVE for streaming mode so can
+		 * have a SVE formatted context with a zero VL and no
+		 * payload data.
+		 */
+		if (!system_supports_sve() && !system_supports_sme())
 			return -EINVAL;
 
 		vl = task_get_sve_vl(current);