NFS4: Avoid migration loops

NFS4: Avoid migration loops

If a server returns itself as a location while migrating, the client may
end up getting stuck attempting to migrate twice to the same server.  Catch
this by checking if the nfs_client found is the same as the existing
client.  For the other two callers to nfs4_set_client, the nfs_client will
always be ERR_PTR(-EINVAL).

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Orabug: 27679350
(cherry picked from commit 52442f9b11b7e5d4a38d99143011831fd171f8d9)
Signed-off-by: Calum Mackay <calum.mackay@oracle.com>
Reviewed-by: Manjunath Patil <manjunath.b.patil@oracle.com>