Give the device major / minor numbers their own separate 32 bit fields

Give the device major / minor numbers their own separate 32 bit fields
in the kernel.  Change dev_ops to use a RB tree to index major device
numbers and remove the 256 device major number limitation.

Build a dynamic major number assignment feature into dev_ops_add() and
adjust ASR (which already had a hand-rolled one), and MFS to use the
feature.  MFS at least does not require any filesystem visibility to
access its backing device.  Major devices numbers >= 256 are used for
dynamic assignment.

Retain filesystem compatibility for device numbers that fall within the
range that can be represented in UFS or struct stat (which is a single
32 bit field supporting 8 bit major numbers and 24 bit minor numbers).