btrfs: add more __cold annotations

The __cold functions are placed to a special section, as they're
expected to be called rarely. This could help i-cache prefetches or help
compiler to decide which branches are more/less likely to be taken
without any other annotations needed.

Though we can't add more __exit annotations, it's still possible to add
__cold (that's also added with __exit). That way the following function
categories are tagged:

- printf wrappers, error messages
- exit helpers

Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index a8bafed..6af7b58 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -923,7 +923,7 @@
 	return ret;
 }
 
-void btrfs_exit_sysfs(void)
+void __cold btrfs_exit_sysfs(void)
 {
 	sysfs_remove_group(&btrfs_kset->kobj, &btrfs_feature_attr_group);
 	kset_unregister(btrfs_kset);