If root filesystem is multidev btrfs, do not attempt to pass all devices as
kernel root= argument. This results in splitting command line in GRUB due to
embedded newline and even if we managed to quote it, kernel does not know how
to interpret it anyway. Multidev btrfs requires user space device scanning,
so passing single device would not work too.

This still respects user settings GRUB_DISABLE_LINUX_UUID. Not sure what we
should do in this case.

Closes: 45709