btrfs: send: add and use helper to rename current inode when processing refs

Extract the logic to rename the current inode at process_recorded_refs()
into a helper function and use it, therefore removing duplicated logic
and making it easier for an upcoming patch by avoiding yet more duplicated
logic.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>