nfsd: add nfsd_file_{get,put} to 'nfs_to' nfsd_localio_operations

In later a commit LOCALIO must call both nfsd_file_get and
nfsd_file_put to manage extra nfsd_file references.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>