memDesc: change ref as unsigned

core/mem.c

@@ -136,6 +136,7 @@ int unrefPhyPage(paddr_t addr)
     if (!mem) {
         return -1;
     }
+    assert(mem->ref >0);
     mem->ref--;
     if (mem->ref == 0) {
         allocatedPage--;

core/mem.h

@@ -11,7 +11,7 @@ extern uint32_t __ld_kernel_end;
 
 struct memDesc {
     paddr_t phy_addr;
-    long ref;
+    unsigned long ref;
     struct memDesc *next, *prev;
 };