[pnfs] [0/6] Provide support for O_DIRECT

[Dean Hildebrand]

These patches provide support for NFSv41/pNFS with O_DIRECT.

Patches 5/6 essentially use ifdef to insert pnfs code into
the existing direct read/write functions.  I'm not entirely
pleased with this, but these are such monolithic functions
that duplicating them for pNFS seems cumbersome.

Note: This patch provides prelim support for non-file layout
drivers, but this should be tested.

1. enables sessions with O_DIRECT.
2. o_direct code reorg to prepare for pnfs
3. fix a bug and align try_to_commit with the other 2 try_to_xxx funcs
4/5/6 o_direct support