Mini-OS: netfront: Fix rx ring starvation in network_rx

Mini-OS: netfront: Fix rx ring starvation in network_rx

In network_rx() we must push the same amount of requests back onto the
ring in the second loop that we consumed in the first loop. Otherwise
the ring will eventually starve itself of free request slots and no
packets will be delivered.

Further, we make the HAVE_LIBC codepath clearer to follow by removing
the "some" variable from the for loop initialisation and conditions.

Signed-off-by: Martin Lucina <martin@lucina.net>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>