nfsd: remove set but not used variable 'len'

Fixes gcc '-Wunused-but-set-variable' warning:

fs/nfsd/nfs4xdr.c: In function nfsd4_encode_splice_read:
fs/nfsd/nfs4xdr.c:3464:7: warning: variable len set but not used [-Wunused-but-set-variable]

It is not used since commit 83a63072 ("nfsd: fix nfs read eof detection")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4xdr.c

@@ -3461,7 +3461,6 @@ static __be32 nfsd4_encode_splice_read(
 	struct xdr_stream *xdr = &resp->xdr;
 	struct xdr_buf *buf = xdr->buf;
 	u32 eof;
-	long len;
 	int space_left;
 	__be32 nfserr;
 	__be32 *p = xdr->p - 2;
@@ -3470,7 +3469,6 @@ static __be32 nfsd4_encode_splice_read(
 	if (xdr->end - xdr->p < 1)
 		return nfserr_resource;
 
-	len = maxcount;
 	nfserr = nfsd_splice_read(read->rd_rqstp, read->rd_fhp,
 				  file, read->rd_offset, &maxcount, &eof);
 	read->rd_length = maxcount;