NFSv4.1: Ensure that the layout sequence id stays 'close' to the current

Clamp the layout barrier sequence id to the current sequence id
minus the maximum number of outstanding layoutget requests.

Also ensure that we correctly initialise lo->plh_barrier if there are
no layout segments associated to this layout header.

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