[SPARK-27868][CORE][FOLLOWUP] Recover the default value to -1 again

The default value for backLog set back to -1, as any other value may break existing configuration by overriding Netty's default io.netty.util.NetUtil#SOMAXCONN. The documentation accordingly adjusted.
See discussion thread: #24732

### What changes were proposed in this pull request?
Partial rollback of #24732 (default for backLog set back to -1).

### Why are the changes needed?
Previous change introduces backward incompatibility by overriding default of Netty's `io.netty.util.NetUtil#SOMAXCONN`

Closes #27230 from xCASx/master.

Authored-by: Maxim Kolesnikov <[email protected]>
Signed-off-by: Marcelo Vanzin <[email protected]>

Author: xCASx

common/network-common/src/main/java/org/apache/spark/network/util/TransportConf.java

@@ -108,8 +108,12 @@ public int numConnectionsPerPeer() {
     return conf.getInt(SPARK_NETWORK_IO_NUMCONNECTIONSPERPEER_KEY, 1);
   }
 
-  /** Requested maximum length of the queue of incoming connections. Default is 64. */
-  public int backLog() { return conf.getInt(SPARK_NETWORK_IO_BACKLOG_KEY, 64); }
+  /**
+   * Requested maximum length of the queue of incoming connections. If  < 1,
+   * the default Netty value of {@link io.netty.util.NetUtil#SOMAXCONN} will be used.
+   * Default to -1.
+   */
+  public int backLog() { return conf.getInt(SPARK_NETWORK_IO_BACKLOG_KEY, -1); }
 
   /** Number of threads used in the server thread pool. Default to 0, which is 2x#cores. */
   public int serverThreads() { return conf.getInt(SPARK_NETWORK_IO_SERVERTHREADS_KEY, 0); }

docs/configuration.md

@@ -844,13 +844,14 @@ Apart from these, the following properties are also available, and may be useful
 </tr>
 <tr>
   <td><code>spark.shuffle.io.backLog</code></td>
-  <td>64</td>
+  <td>-1</td>
   <td>
     Length of the accept queue for the shuffle service. For large applications, this value may
     need to be increased, so that incoming connections are not dropped if the service cannot keep
     up with a large number of connections arriving in a short period of time. This needs to
     be configured wherever the shuffle service itself is running, which may be outside of the
-    application (see <code>spark.shuffle.service.enabled</code> option below).
+    application (see <code>spark.shuffle.service.enabled</code> option below). If set below 1,
+    will fallback to OS default defined by Netty's <code>io.netty.util.NetUtil#SOMAXCONN</code>.
   </td>
 </tr>
 <tr>