rseq/selftests: Fix namespace collision with rseq UAPI header

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: e6644c967d3c ("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