NFSv4.1/pnfs: Ensure we handle the error NFS4ERR_RETURNCONFLICT

NFSv4.1/pnfs: Ensure we handle the error NFS4ERR_RETURNCONFLICT

Once the client has processed the CB_LAYOUTRECALL, but has not yet
successfully returned the layout, the server is supposed to switch to
returning NFS4ERR_RETURNCONFLICT. This patch ensures that we handle
that return value correctly.

Fixes: 183d9e7b112a ("pnfs: rework LAYOUTGET retry handling")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>