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>