nfsv4: set hostname when creating nfsv4 ds connection

We reference cl_hostname in many places for debugging purpose.
So make it useful by setting hostname when calling nfs_get_client.

Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>