Fix build with KASAN

The stock zstd code expects some helpers from ASAN if present.
This works fine in userland, but in kernel, KASAN also gets detected,
and lacks those helpers. So let's make some empty substitutes for
that case.

Signed-off-by: Rich Ercolani <rincebrain@gmail.com>

module/zstd/zfs_zstd.c

@@ -202,6 +202,25 @@ static struct zstd_fallback_mem zstd_dctx_fallback;
 static struct zstd_pool *zstd_mempool_cctx;
 static struct zstd_pool *zstd_mempool_dctx;
 
+/*
+ * The library zstd code expects these if ADDRESS_SANITIZER gets defined,
+ * and while ASAN does this, KASAN defines that and does not. So to avoid
+ * changing the external code, we do this.
+ */
+#if defined(__has_feature)
+#if __has_feature(address_sanitizer)
+#define	ADDRESS_SANITIZER 1
+#endif
+#elif defined(__SANITIZE_ADDRESS__)
+#define	ADDRESS_SANITIZER 1
+#endif
+#if defined(_KERNEL) && defined(ADDRESS_SANITIZER)
+void __asan_unpoison_memory_region(void const volatile *addr, size_t size);
+void __asan_poison_memory_region(void const volatile *addr, size_t size);
+void __asan_unpoison_memory_region(void const volatile *addr, size_t size) {};
+void __asan_poison_memory_region(void const volatile *addr, size_t size) {};
+#endif
+
 
 static void
 zstd_mempool_reap(struct zstd_pool *zstd_mempool)