Add a precheck before the actual call to fcntl (#8360)

Don't bother to call fcntl if the flags are not gonna be changed.

src/anet.c

@@ -69,6 +69,11 @@ int anetSetBlock(char *err, int fd, int non_block) {
         return ANET_ERR;
     }
 
+    /* Check if this flag has been set or unset, if so, 
+     * then there is no need to call fcntl to set/unset it again. */
+    if (!!(flags & O_NONBLOCK) == !!non_block)
+        return ANET_OK;
+
     if (non_block)
         flags |= O_NONBLOCK;
     else