NFS: Fix a compile issue when CONFIG_NFS_V4_1 is undefined

struct nfs_direct_req can't compile when struct pnfs_ds_commit_info
is undefined.

Reported-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Fred Isaman <iisaman@netapp.com>