NFS: Reduce stack use in encode_exchange_id()

encode_exchange_id() uses more stack space than necessary, giving a compile
time warning. Reduce the size of the static buffer for implementation name.

Signed-off-by: Jim Rees <rees@umich.edu>
Reviewed-by: "Adamson, Dros" <Weston.Adamson@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>