initramfs-init can't ro mount zfs root at /sysroot

I upgraded from v3.12.1 to edge (zfs 0.8.4-r0 -> 2.0.0-r0) to see what I'd hit before v3.13.0 release and caught this.

Here we default to mounting /sysroot read-only initramfs-init.in#L510, which yields "failed: Resource busy".

Manually running mount(8) with -t zfs -o rw works and the system will boot when you exit the rescue shell. For now I have added rootflags=rw to my cmdline as a workaround but that doesn't feel right.