luksFormat hangs if udev is stopped

Encrypted RAID devices that are not created from snapshots are
consistently hanging on the `cryptsetup luksFormat`
step. Experimentation showed that restarting the udev exec queue caused
luksFormat to resume.

AFAIK we only need to disable udev events until after the md device has
been assembled, so we now start it back up immediately after creating
the RAID device.