Change the check to be <= instead of <. This fixes one occurrence of an
apparently widespread division-by-zero bug in our TCP code: if a user adds
huge IPv6 options with setsockopt, and if the total size of the options
happens to be equal to the available space calculated for the TCP payload,
t_segsz gets set to zero, and given that we then divide several things by
it, the kernel crashes.

(maxv)