block: Use a GString in bdrv_perm_names()

This is a bit more efficient than having to allocate and free memory
for each new permission.

The default size (30) is enough for "consistent read, write, resize".
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Message-id: 20200110171518.22168-1-berto@igalia.com
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block.c

@@ -1998,18 +1998,19 @@ char *bdrv_perm_names(uint64_t perm)
         { 0, NULL }
     };
 
-    char *result = g_strdup("");
+    GString *result = g_string_sized_new(30);
     struct perm_name *p;
 
     for (p = permissions; p->name; p++) {
         if (perm & p->perm) {
-            char *old = result;
-            result = g_strdup_printf("%s%s%s", old, *old ? ", " : "", p->name);
-            g_free(old);
+            if (result->len > 0) {
+                g_string_append(result, ", ");
+            }
+            g_string_append(result, p->name);
         }
     }
 
-    return result;
+    return g_string_free(result, FALSE);
 }
 
 /*