NFS: Force server to drop NFSv4 state

nfs4_reset_all_state() refreshes the boot verifier a server sees to
trigger that server to wipe this client's state.  This function is
invoked when an NFSv4.1 server reports that it has revoked some or
all of a client's NFSv4 state.

To facilitate server trunking discovery, we will eventually want to
move the cl_boot_time field to a more global structure.  The Uniform
Client String model (and specifically, server trunking detection)
requires that all servers see the same boot verifier until the client
actually does reboot, and not a fresh verifier every time the client
unmounts and remounts the server.

Without the cl_boot_time field, however, nfs4_reset_all_state() will
have to find some other way to force the server to purge the client's
NFSv4 state.

Because these verifiers are opaque (ie, the server doesn't know or
care that they happen to be timestamps), we can force the server
to wipe NFSv4 state by updating the boot verifier as we do now, then
immediately afterwards establish a fresh client ID using the old boot
verifier again.

Hopefully there are no extra paranoid server implementations that keep
track of the client's boot verifiers and prevent clients from reusing
a previous one.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

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