NFSD: Simplify starting_len

[ Upstream commit 071ae99f ]

Clean-up: Now that nfsd4_encode_readv() does not have to encode the
EOF or rd_length values, it no longer needs to subtract 8 from
@starting_len.

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