NFS: Remove unused authflavour parameter from nfs_get_client()

This parameter hasn't been used since f8407299 (Linux 3.11-rc2), so
let's remove it from this function and callers.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>