Invoke the newly introduced nsm_reboot_lookup() function in
nlm_host_rebooted() instead of nsm_find().

This introduces just one behavioral change: debugging messages
produced during reboot notification will now appear when the
NLMDBG_MONITOR flag is set, but not when the NLMDBG_HOSTCACHE flag
is set.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>