LU-50 ldiskfs: pdirops patch for ldiskfs

LU-50 ldiskfs: pdirops patch for ldiskfs

Single directory performance is a critical for HPC workloads. In a
typical use case an application creates a separate output file for
each node and task in a job. As nodes and tasks increase, hundreds
of thousands of files may be created in a single directory within
a short window of time.
Today, both filename lookup and file system modifying operations
(such as create and unlink) are protected with a single lock for
an entire ldiskfs directory. PDO project will remove this
bottleneck by introducing a parallel locking mechanism for entire
ldiskfs directories. This work will enable multiple application
threads to simultaneously lookup, create and unlink in parallel.

This patch contains:
  - pdirops support for ldiskfs
  - N-level htree directory
  - integrate with osd-ldiskfs

Change-Id: I269c0e3112e68f3acd79e860dab052a68c7d7aaa
Signed-off-by: Liang Zhen <liang@whamcloud.com>