btrfs: avoid linker error in btrfs_find_create_tree_block()

[ Upstream commit 7ef3cbf17d2734ca66c4ed8573be45f4e461e7ee ]

The inline function btrfs_is_testing() is hardcoded to return 0 if
CONFIG_BTRFS_FS_RUN_SANITY_TESTS is not set. Currently we're relying on
the compiler optimizing out the call to alloc_test_extent_buffer() in
btrfs_find_create_tree_block(), as it's not been defined (it's behind an
 #ifdef).

Add a stub version of alloc_test_extent_buffer() to avoid linker errors
on non-standard optimization levels. This problem was seen on GCC 14
with -O0 and is helps to see symbols that would be otherwise optimized
out.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/btrfs/extent_io.c

@@ -3565,10 +3565,10 @@ struct extent_buffer *find_extent_buffer(struct btrfs_fs_info *fs_info,
 	return eb;
 }
 
-#ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
 struct extent_buffer *alloc_test_extent_buffer(struct btrfs_fs_info *fs_info,
 					u64 start)
 {
+#ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
 	struct extent_buffer *eb, *exists = NULL;
 	int ret;
 
@@ -3604,8 +3604,11 @@ again:
 free_eb:
 	btrfs_release_extent_buffer(eb);
 	return exists;
-}
+#else
+	/* Stub to avoid linker error when compiled with optimizations turned off. */
+	return NULL;
 #endif
+}
 
 static struct extent_buffer *grab_extent_buffer(
 		struct btrfs_fs_info *fs_info, struct page *page)