slirp: fix clearing ifq_so from pending packets

The if_fastq and if_batchq contain not only packets, but queues of packets
for the same socket. When sofree frees a socket, it thus has to clear ifq_so
from all the packets from the queues, not only the first.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>