slirp: Prevent recursion of if_start

if_start can be called recursively via if_encap. Avoid this as our
scheme of dequeuing packets is not compatible with this.

CC: Fabien Chouteau <chouteau@adacore.com>
CC: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
CC: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

slirp/slirp.h

@@ -239,6 +239,7 @@ struct Slirp {
     struct mbuf if_fastq;   /* fast queue (for interactive data) */
     struct mbuf if_batchq;  /* queue for non-interactive data */
     struct mbuf *next_m;    /* pointer to next mbuf to output */
+    bool if_start_busy;     /* avoid if_start recursion */
 
     /* ip states */
     struct ipq ipq;         /* ip reass. queue */