[netdevice] Process all received packets in net_poll()

[netdevice] Process all received packets in net_poll()

The current logic is to process at most one received packet per call
to net_poll(), on the basis that refilling the hardware descriptor
ring should be delayed as little as possible.  However, this limits
the rate at which packets can be processed and ultimately ends up
adding latency which, in turn, limits the achievable throughput.

With temporary modifications in place to essentially remove all
resource constraints (heap size increased to 16MB, RX descriptor ring
increased to 64 descriptors) and a TCP window size of 1MB, the
throughput on a gigabit (i.e. 119MBps) network can be observed to fall
off exponentially from around 115MBps to around 75MBps.  Changing
net_poll() to process all received packets results in a steady
119MBps throughput.

Signed-off-by: Michael Brown <mcb30@ipxe.org>