nfsd: add support for delegated timestamps

nfsd: add support for delegated timestamps

Add support for the delegated timestamps on write delegations. This
allows the server to proxy timestamps from the delegation holder to
other clients that are doing GETATTRs vs. the same inode.

When OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS bit is set in the OPEN
call, set the dl_type to the *_ATTRS_DELEG flavor of delegation.

Add timespec64 fields to nfs4_cb_fattr and decode the timestamps into
those. Vet those timestamps according to the delstid spec and update
the inode attrs if necessary.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>