btrfs: Open-code add_delayed_data_ref

Now that the initialization part and the critical section code have been
split it's a lot easier to open code add_delayed_data_ref. Do so in the
following manner:

1. The common init function is put immediately after memory-to-be-initialized
   is allocated, followed by the specific data ref initialization.

2. The only piece of code that remains in the critical section is
   insert_delayed_ref call.

3. Tracing and memory freeing code is moved outside of the critical
   section.

No functional changes, just an overall shorter critical section.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>