Comment # 4 on bug 912170 from Andrei Borzenkov

(In reply to Ryan Kingsbury from comment #3)
> Andrei,
> 
> THANK YOU! Your fix worked. Will this workaround survive future updates to
> the kernel or udev?
> 

Yes, it will; but it makes sense to compare updated file and merge back any
other change.

@systemd-maintaners: note that this causes failure to mount btrfs even
*MANUALLY*. Because patch effectively skips calling "btrfs ready" for every
device-mapper device, when you run "mount /dev/xxx" kernel knows only about one
single device /dev/xxx and fails mount. User needs to either run "btrfs scan"
before or repeat mount for every device that is part of multi-device btrfs.