nfs: factor out {encode,decode}_opaque_fixed to nfs_xdr.h

Eliminates duplicate functions in various files to allow for
additional callers.

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>