NFS: Split out NFS v4 inode operations

The NFS v4 file inode operations are already already in nfs4proc.c, so
this patch just needs to move the directory operations to the same file.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>