[SPARK-3889] Attempt to avoid SIGBUS by not mmapping files in ConnectionManager

In general, individual shuffle blocks are frequently small, so mmapping them
often creates a lot of waste. It may not be bad to mmap the larger ones, but
it is pretty inconvenient to get configuration into ManagedBuffer, and besides
it is unlikely to help all that much.

Note that user of ManagedBuffer#nioByteBuffer() seems generally bad practice,
and would ideally never be used for data that may be large. Users of such data
would ideally stream the data instead.

Author: aarondav

core/src/main/scala/org/apache/spark/network/ManagedBuffer.scala

@@ -72,7 +72,10 @@ final class FileSegmentManagedBuffer(val file: File, val offset: Long, val lengt
     var channel: FileChannel = null
     try {
       channel = new RandomAccessFile(file, "r").getChannel
-      channel.map(MapMode.READ_ONLY, offset, length)
+      val buf = ByteBuffer.allocate(length.toInt)
+      channel.read(buf, offset)
+      buf.flip()
+      buf
     } catch {
       case e: IOException =>
         Try(channel.size).toOption match {