lockd: release reference to nsm_handle in nlm_host_rebooted

nsm_reboot_lookup takes a reference to the nsm_handle that it returns,
but nlm_host_rebooted never releases that reference.

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