fs-backend: Fix freelist implementation

diff -r 064c96848e2f tools/fs-back/fs-backend.c
@@ -110,9 +110,9 @@
     unsigned short *freelist;
     
     requests = malloc(sizeof(struct fs_request) *nr_entries);
-    freelist = malloc(sizeof(unsigned short) * nr_entries); 
+    freelist = malloc(sizeof(unsigned short) * (nr_entries + 1)); 
     memset(requests, 0, sizeof(struct fs_request) * nr_entries);
-    memset(freelist, 0, sizeof(unsigned short) * nr_entries);
+    memset(freelist, 0, sizeof(unsigned short) * (nr_entries + 1));
     for(i=0; i< nr_entries; i++)
     {
         requests[i].active = 0; 

@@ -72,14 +72,14 @@
 
 static inline void add_id_to_freelist(unsigned int id,unsigned short* freelist)
 {
-    freelist[id] = freelist[0];
+    freelist[id + 1] = freelist[0];
     freelist[0]  = id;
 }
 
 static inline unsigned short get_id_from_freelist(unsigned short* freelist)
 {
     unsigned int id = freelist[0];
-    freelist[0] = freelist[id];
+    freelist[0] = freelist[id + 1];
     return id;
 }