lockd: Remove unused fields in the nlm_reboot structure

The nlm_reboot structure is used to store information provided by the
NSM_NOTIFY procedure.  This procedure is not specified by the NLM or NSM
protocols, other than to say that the procedure can be used to transmit
information private to a particular NLM/NSM implementation.

For Linux, the callback arguments include the name of the monitored host,
the new NSM state of the host, and a 16-byte private opaque.

As a clean up, remove the unused fields and the server-side XDR logic that
decodes them.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>