netfront.c: avoid gcc variably-modified warning

gcc produces a "variably modified X at file scope" warning for
structures that use these size definitions. I think the definitions are
actually fine but can be rephrased with the __CONST_RING_SIZE macro more
cleanly anyway.

Reviewed by: markj, royger
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon
Differential revision: https://reviews.freebsd.org/D11417