tcp: bump max snd buffer size for autoscaling

This allows in higher throughput values with default settings.
In the review I was proposing using 16 MB, but in the transport
call today we settled on a more conservative value of 8.
Bumping it further will be done in combination with mitigations
for mbuf exhaustion attacks.

Reviewed by: rscheff, cc, glebius, Nick Banks, Peter Lei, jtl, thj, rrs
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52872