kcsan, compiler_types: avoid duplicate type issues in BPF Type Format

Enabling KCSAN is causing a large number of duplicate types in BTF for
core kernel structs like task_struct [1]. This is due to the definition
in include/linux/compiler_types.h

`#ifdef __SANITIZE_THREAD__
...
`#define __data_racy volatile
..
`#else
...
`#define __data_racy
...
`#endif

Because some objects in the kernel are compiled without KCSAN flags
(KCSAN_SANITIZE) we sometimes get the empty __data_racy annotation for
objects; as a result we get multiple conflicting representations of the
associated structs in DWARF, and these lead to multiple instances of core
kernel types in BTF since they cannot be deduplicated due to the
additional modifier in some instances.

Moving the __data_racy definition under CONFIG_KCSAN avoids this problem,
since the volatile modifier will be present for both KCSAN and
KCSAN_SANITIZE objects in a CONFIG_KCSAN=y kernel.

Link: https://lkml.kernel.org/r/20260116091730.324322-1-alan.maguire@oracle.com
Fixes: 31f605a308e6 ("kcsan, compiler_types: Introduce __data_racy type qualifier")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Reported-by: Nilay Shroff <nilay@linux.ibm.com>
Tested-by: Nilay Shroff <nilay@linux.ibm.com>
Suggested-by: Marco Elver <elver@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Bart van Assche <bvanassche@acm.org>
Cc: Daniel Borkman <daniel@iogearbox.net>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jason A. Donenfeld <jason@zx2c4.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Naman Jain <namjain@linux.microsoft.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stanislav Fomichev <sdf@fomichev.me>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>