mbuf: Remove UDP_IPV4_EX, which is never defined.

[sepherosa_gmail.com]

Summary

Add comment to explain the IPV6_EX suffix.  The confusion about
these RSS hash type probably stems from the facts that they were
never widely implemented by hardwares.

[rwatson]

This seems generally sensible.

I'm not quite sure about keeping the "Non-standard RSS hash types" comment, as M_HASHTYPE_OPAQUE is basically intended to be a catch-all for any other model, and is not specifically intended to be RSS (hence not having RSS in its name). So I'd be tempted to drop that comment.

[sepherosa_gmail.com]

In D12453#258240, @rwatson wrote:

This seems generally sensible.

I'm not quite sure about keeping the "Non-standard RSS hash types" comment, as M_HASHTYPE_OPAQUE is basically intended to be a catch-all for any other model, and is not specifically intended to be RSS (hence not having RSS in its name). So I'd be tempted to drop that comment.

OK, sure.