ixl: Fix mbuf hash type settings.

IPV6_EXs in RSS never mean fragment. They mean:
"- Home address from the home address option in the IPv6 destination

options header.  If the extension header is not present, use the
Source IPv6 Address.

• IPv6 address that is contained in the Routing-Header-Type-2 from the associated extension header. If the extension header is not present, use the Destination IPv6 Address."

UDP_IPV4_EX is an invalid RSS hash type, which will be removed.

Quoted from:
https://docs.microsoft.com/en-us/windows-hardware/drivers/network/rss-hashing-types#ndishashipv6ex

Reviewed by: erj
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D12450