NFSv4.1: filelayout driver specific code for COMMIT

Implement all the hooks created in the previous patches.
This requires exporting quite a few functions and adding a few
structure fields.

Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>