Wanted to do this a while back but completely forgot.
This makes shm handling simpler as the compositor doesn't have to worry about the SIGBUS handler.
See sigbus_is_impossible in https://gitlab.freedesktop.org/wayland/wayland/-/blob/main/src/wayland-shm.c#L325
This would have avoided phoc#290 (closed) as well (but it's good to have that sorted on the compositor side so clients can't trigger it). https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/474 is a good reference.
I added a warning for linux without memfd_create.
I got reminded of this by https://lwn.net/Articles/918106/