I get a lengthy error while trying to compile the shiftfs module for kernel 5.16.
I am using the shiftfs file from the jammy ubuntu kernel repo:
https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/jammy/log/fs/shiftfs.c?h=master-next
I can provide the log if necessary, but I am quite sure you can reproduce.