nfs: pass struct nfsd_file to nfs_init_pgio and nfs_init_commit

The nfsd_file will be passed, in future commits, by callers
that enable LOCALIO support (for both regular NFS and pNFS IO).

[Derived from patch authored by Weston Andros Adamson, but switched
 from passing struct file to struct nfsd_file]

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>