slirp: Fix intermittent send queue hangs on a socket

slirp: Fix intermittent send queue hangs on a socket

if_output() originally sent one mbuf per call and used the slirp->next_m
variable to keep track of where it left off.  But nowadays it tries to
send all of the mbufs from the fastq, and one mbuf from each session on
the batchq.  The next_m variable is both redundant and harmful: there is
a case[0] involving delayed packets in which next_m ends up pointing
to &slirp->if_batchq when an active session still exists, and this
blocks all traffic for that session until qemu is restarted.

The test case was created to reproduce a problem that was seen on
long-running Chromium OS VM tests[1] which rapidly create and
destroy ssh connections through hostfwd.

[0] https://pastebin.com/NNy6LreF
[1] https://bugs.chromium.org/p/chromium/issues/detail?id=766323

Signed-off-by: Kevin Cernekee <cernekee@chromium.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>