Fix build with KASAN

The stock zstd code expects some helpers from ASAN if ADDRESS_SANITIZER
is defined. This works fine in userland, but in kernel, KASAN also
defines that but not 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 +202,18 @@ 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(_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)