NFSv4: Fix issues in nfs4_discover_server_trunking

- Ensure that we exit with ENOENT if the call to ops->get_clid_cred()
  fails.
- Handle the case where ops->detect_trunking() exits with an
  unexpected error, and return EIO.

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