NFSv4/pNFS: Do layout state recovery upon reboot

Some pNFS implementations, such as flexible files, want the client to
send the layout stats and layout errors that may have incurred while the
metadata server was booting. To do so, the client sends a layoutreturn
with an all-zero stateid while the server is in grace during reboot
recovery.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>