NFSD: Clean up nfsd4_encode_readlink()

[ Upstream commit 99b002a1 ]

Similar changes to nfsd4_encode_readv(), all bundled into a single
patch.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>