rseq/selftests: Fix namespace collision with rseq UAPI header

When the rseq UAPI header is included, 'union rseq' clashes with 'struct
rseq'. It's not the case in the rseq selftests but it does break the KVM
selftests that also include this file.

Rename 'union rseq' to 'union rseq_tls' to fix this.

Fixes: e6644c9 ("rseq/selftests: Ensure the rseq ABI TLS is actually 1024 bytes")
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/r/20250319202144.1141542-1-mjeanson@efficios.com

Author: mjeanson

tools/testing/selftests/rseq/rseq.c

@@ -75,13 +75,13 @@ static int rseq_ownership;
  * Use a union to ensure we allocate a TLS area of 1024 bytes to accomodate an
  * rseq registration that is larger than the current rseq ABI.
  */
-union rseq {
+union rseq_tls {
 	struct rseq_abi abi;
 	char dummy[RSEQ_THREAD_AREA_ALLOC_SIZE];
 };
 
 static
-__thread union rseq __rseq __attribute__((tls_model("initial-exec"))) = {
+__thread union rseq_tls __rseq __attribute__((tls_model("initial-exec"))) = {
 	.abi = {
 		.cpu_id = RSEQ_ABI_CPU_ID_UNINITIALIZED,
 	},