Move a locking function that was spread throughout disk device drivers into

Move a locking function that was spread throughout disk device drivers into
the disk structure.

The locking was mostly used in disk device drivers to prevent multiple
disklabel operations from going on simultaneously against the device.

Added disk_construct(), a constructor for the disk structure, that
initializes the lock structure in disk. It is called by default in
disk_attach() if it hasn't been called yet.

Added disk_lock and disk_unlock