tcp: Unifidef use of rss software hash in syncache

Ever since "d9c55b2e8cd6 rss: Enable portions of RSS globally.."
exposed the RSS software hashing functions, it has been possible
to use them without "ifdef RSS". Do so now in the syncache
so as to get flowids recorded.

Note that the use of the rss hash functions is conditional on IP versions,
so we must ifdef INET to ensure rss_proto_software_hash_v4() is available.

Fixes 73fe85e486d2
Sponsored by: Netflix
Reviewed by: glebius, p.mousavizadeh_protonmail.com, nickbanks_netflix.com, tuexen
Differential Revision: https://reviews.freebsd.org/D54534