nfsd: add __force to opaque verifier field casts

sparse complains that we're stuffing non-byte-swapped values into
__be32's here. Since they're supposed to be opaque, it doesn't matter
much. Just add __force to make sparse happy.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>