NFSD: Clean up WRITE arg decoders

[ Upstream commit d4da5baa ]

xdr_stream_subsegment() already returns a boolean value.

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