changes to get test #46 working

- bunch of fixes to handle dir splitting over N < MDS nodes
  - fixes to MDS code to handle dir splitting over N < MDS nodes properly
  - mds_try_to_split() should ignore requests to split dir over 1 node
  - checks for NULL exports in LMV
  - checks for wrong -ERESTART handling: because of software errors
    it possible that lmv will loop forever. we need to assert on this.
  - one more sanity test 3c to test lfs dirstripe and activity on such a dirs