sfxge(4): support equal stride super-buffer Rx mode

Equal stride super-buffer Rx mode is supported by DPDK firmware
variant. One Rx descriptor provides many Rx buffers to firmware.
Rx buffers follow each other with specified stride.
Also it supports head of line blocking with timeout to address
drops when no Rx descriptors are available. So it gives extra time
to the driver to provide Rx descriptors before drop.

Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18236