NFSv4.1: Use layout credentials for get_deviceinfo calls

This is not strictly needed, since get_deviceinfo is not allowed to
return NFS4ERR_ACCESS or NFS4ERR_WRONG_CRED, but lets do it anyway
for consistency with other pNFS operations.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>