NFS: Clean up nfs_init_client()

We always call nfs_mark_client_ready() even if nfs_create_rpc_client()
returns an error, so we can rearrange nfs_init_client() to mark the
client ready from a single place.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>