crypt: Wait for udev to settle before unlocking disk

crypt: Wait for udev to settle before unlocking disk

To eliminate a race condition that occurs when unlocking one device
depends on the result of unlocking a device before it, the crypt
module must wait for udev to settle between each unlock attempt.

Example

/etc/crypttab:

  keyfile /dev/md1 none luks
  sda4_crypt /dev/sda4 /dev/mapper/keyfile luks
  sdb4_crypt /dev/sdb4 /dev/mapper/keyfile luks

Without this patch, sometimes /dev/sda4 fails to unlock because udev
doesn't have time to create /dev/mapper/keyfile before it's needed.