netinet: Use ICMP_REDIRECT_DEFAULT_TIMEOUT for redirtimeout and clarify sysctl description

[amy.vargas_netapp.com]

Summary

Replace the hardcoded default value for the ICMP redirect route timeout sysctl (redirtimeout) with the ICMP_REDIRECT_DEFAULT_TIMEOUT macro for improved maintainability. Also, clarify the sysctl description to explicitly refer to "ICMP redirect route" instead of "redirect route".

[glebius]

But where is the ICMP_REDIRECT_DEFAULT_TIMEOUT defined? The patch as is can't be compilable.

[amy.vargas_netapp.com]

We are still working out some kinks on our end end to ensure this does not happen again. Once that is all resolved, I will resubmit under a new differential that has all the appropriate changes.