staging: lustre: prevent underflows in lnet_parse_reply()

I don't know if "hdr->payload_length;" can really be negative, but if so
then we shouldn't allow it.  Do the comparison as an unsigned.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>