Update rss_mbuf_software_hash_v4() to check if the mbuf already has
a suitable flowid to use.

It's a bit complicated and nasty because the TCP and UDP hash types
may be actually disabled, falling back to IPv4 hashing.

By default UDP is actually this - a correctly written RSS driver
will currently be disabling IPv4 UDP 4-tuple hashing and instead
IPv4 UDP frames are hashed as 2-tuple IPv4 frames.