NFS: create struct nfs_commit_info

It is COMMIT that is handled the most differently between
the paged and direct paths.  Create a structure that encapsulates
everything either path needs to know about the commit state.

We could use void to hide some of the layout driver stuff, but
Trond suggests pulling it out to ensure type checking, given the
huge changes being made, and the fact that it doesn't interfere
with other drivers.

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