NFS: nfs41_walk_client_list(): re-lock before iterating

Sparse identified an execution path in nfs41_walk_client_list()
where the nfs_client_lock is not re-acquired before taking the next
loop iteration.

fs/nfs/nfs4client.c:437:9: sparse: context imbalance in
 'nfs41_walk_client_list' - different lock contexts for basic block
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4client.c

@@ -450,6 +450,7 @@ int nfs41_walk_client_list(struct nfs_client *new,
 			error = nfs_wait_client_init_complete(pos);
 			if (error < 0) {
 				nfs_put_client(pos);
+				spin_lock(&nn->nfs_client_lock);
 				continue;
 			}