pNFS/NFSv4: Update the layout barrier when we schedule a layoutreturn

When we're scheduling a layoutreturn, we need to ignore any further
incoming layouts with sequence ids that are going to be affected by the
layout return.

Fixes: 44ea8dfc

 ("NFS/pnfs: Reference the layout cred in pnfs_prepare_layoutreturn()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>