lockd: reorganize nlm_host_rebooted

Minor reorganization; no change in behavior.  This will save some
duplicated code after we split the client and server host caches.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
[ cel: Forward-ported to 2.6.37 ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>