Bug in ICMP redirect handling

[prabhakar lakhera]

This seems like a bug:

https://svnweb.freebsd.org/base/head/sys/netinet/tcp_subr.c?r1=303457&r2=303456&pathrev=303457

I believe the ordering of if/else checks for cmd value is important as the
last one checks for mapping of PRC* to an error.

Given that the mapping is defined to be 0 for redirects (only cached routes
are supposed to be invalidated), looks like right now we'll simply return
from:

else if ((unsigned)cmd >= PRC_NCMDS || *inetctlerrmap[cmd] == 0)* return;

The above is I guess is only in the head and not in a major version yet.