Grant Netty permission to read system somaxconn

When Netty listens on a socket, it specifies the established connection
backlog for the socket. On Linux, Netty tries to read the system-wide
configuration for this from /proc/sys/net/core/somaxconn and falls back
to a default value when it can not read this value. This commit grants
Netty permission to read this file so that it can honor the system-wide
configuration for the connection backlog for sockets that it is
listening on. This also removes an obnoxious stack trace that appears
when Netty logging is set to debug logging.

Relates #21840

modules/transport-netty4/src/main/plugin-metadata/plugin-security.policy

@@ -17,8 +17,13 @@
  * under the License.
  */
 
+grant codeBase "${codebase.netty-common-4.1.6.Final.jar}" {
+   // for reading the system-wide configuration for the backlog of established sockets
+   permission java.io.FilePermission "/proc/sys/net/core/somaxconn", "read";
+};
+
 grant codeBase "${codebase.netty-transport-4.1.6.Final.jar}" {
    // Netty NioEventLoop wants to change this, because of https://bugs.openjdk.java.net/browse/JDK-6427854
    // the bug says it only happened rarely, and that its fixed, but apparently it still happens rarely!
    permission java.util.PropertyPermission "sun.nio.ch.bugLevel", "write";
-};
+};