btrfs: send: introduce recorded_ref_alloc and recorded_ref_free

Introduce wrappers to allocate and free recorded_ref structures.

Reviewed-by: Robbie Ko <robbieko@synology.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: BingJing Chang <bingjingc@synology.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>