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>

fs/btrfs/extent_map.c

@@ -19,7 +19,7 @@ int __init extent_map_init(void)
 	return 0;
 }
 
-void extent_map_exit(void)
+void __cold extent_map_exit(void)
 {
 	kmem_cache_destroy(extent_map_cache);
 }