NFS: Use NFS4_MAX_SESSIONID_LEN directly for decode/encode sessionid

It's no need to define a temporary variables for NFS4_MAX_SESSIONID_LEN.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>