LU-993 osd: code cleanup for directory nlink count

- LDISKFS_LINK_MAX is ldiskfs unique, we should not handle it in the mdd
  layer. It's moved down to the osd layer in this patch.

- Remove the declared operation count assertion in OSD_EXEC_OP(), since
  the undo operations are not declared, if there some operation failed,
  the undo operation will trigger the assertion unnecessarily.

- Restore the sub-directory count to 70000 for the sanity test_51b, and
  add test_51ba for testing unlink > LDISKFS_LINK_MAX sub-directories.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: Id02e7d2ac3a7664a55566b1de783f0a73162339b
Reviewed-on: http://review.whamcloud.com/1971
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>