NFSv4: Allow the state manager to mark an open_owner as being recovered

This patch adds a seqcount_t lock for use by the state manager to
signal that an open owner has been recovered. This mechanism will be
used by the delegation, open and byte range lock code in order to
figure out if they need to replay requests due to collisions with
lock recovery.

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