Support bcachefs encrypted root unlocking

Just like how "crypt" module opens luks encrypted root via cryptsetup. It would be great if dracut could unlock bcachefs encrypted partitions a similar way. In userspace the way to unlock a bcachefs encrypted drive and mount it is:

bcachefs unlock /dev/sdaX
# type password
mount /dev/sdaX /mountpoint

However, it is a bit different on systemd for some reason where you need to specify a -k value. And from my testing in fedora 40 beta (my current system uses openrc) I had this issue where I had to type the password again when mounting it (could be a bug with fedora 40 beta).

bcachefs unlock -k session /dev/sdaX
# type password
bcachefs mount /dev/sdaX /mountpoint
# type password again

There is more of an explanation on their website https://bcachefs.org/Encryption/.

[Alunderin]

When running:

bcachefs unlock /dev/sdaX
mount /dev/sdaX /mountpoint

When logged in, I only need to enter the password once. When trying to do the same thing in a dracut module, I need to type it twice. The -k session option has no effect on this and the system I'm using does not use systemd.

Based on what breavyn did, I've found that using keyctl to manually move the key from the user keyring to the session keyring seems to allow you to only input the password once.

bcachefs unlock /dev/sdaX
keyctl link @u @s
mount /dev/sdaX /mountpoint

I would have though that -k session would do the same thing, but apparently that's not the case. Linking the two keyrings might also be a security concern, though I don't know anything about kernel keyrings so I can't say for certain.

Here's the dracut module I'm working with: https://github.com/Alunderin/dracut-bcachefs