nfsd uses several different mechanisms for identifying the filesystem
from the filehandle.

This patch:
  Marks type 2 as deprecated - it wastes space, and space in the filehandle
     is not unlimited
  Adds type 3 which handles new, large device number in 32bits of space
  Tidies up the code for determining which type to use in a newly created
  filehandle - the addition of type 2 broke this code.