slirp: Keep next_m always valid

Make sure that next_m always points to a packet if batchq is non-empty.
This will simplify walking the queues in if_start.

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

slirp/if.c

@@ -96,8 +96,13 @@ if_output(struct socket *so, struct mbuf *ifm)
 			ifs_insque(ifm, ifq->ifs_prev);
 			goto diddit;
 		}
-	} else
+        } else {
 		ifq = slirp->if_batchq.ifq_prev;
+                /* Set next_m if the queue was empty so far */
+                if (slirp->next_m == &slirp->if_batchq) {
+                    slirp->next_m = ifm;
+                }
+        }
 
 	/* Create a new doubly linked list for this session */
 	ifm->ifq_so = so;
@@ -170,13 +175,8 @@ void if_start(Slirp *slirp)
         if (slirp->if_fastq.ifq_next != &slirp->if_fastq) {
             ifm = slirp->if_fastq.ifq_next;
         } else {
-            /* Nothing on fastq, see if next_m is valid */
-            if (slirp->next_m != &slirp->if_batchq) {
-                ifm = slirp->next_m;
-            } else {
-                ifm = slirp->if_batchq.ifq_next;
-            }
-
+            /* Nothing on fastq, pick up from batchq via next_m */
+            ifm = slirp->next_m;
             from_batchq = true;
         }
 
@@ -202,6 +202,12 @@ void if_start(Slirp *slirp)
         if (ifm->ifs_next != ifm) {
             insque(ifm->ifs_next, ifqt);
             ifs_remque(ifm);
+            /* Set next_m if the session packet is now the only one on
+             * batchq */
+            if (ifqt == &slirp->if_batchq &&
+                slirp->next_m == &slirp->if_batchq) {
+                slirp->next_m = ifm->ifs_next;
+            }
         }
 
         /* Update so_queued */