when an NFSv4 server reboots, by doing two things.
1 - Make the function that acquires a stateid for I/O operations
block until recovery is complete, so that it doesn't acquire
out of date stateids.
2 - Only allow a recovery once every 1/2 of a lease duration, since
the NFSv4 server must provide a recovery grace period of at
least a lease duration. This should avoid recoveries caused
by an out of date stateid that was acquired for an I/O op.
just before a recovery cycle started.
MFC after: 1 week
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