[SPARK-27259][CORE] Allow setting -1 as length for FileBlock

### What changes were proposed in this pull request?

This PR aims to update the validation check on `length` from `length >= 0` to `length >= -1` in order to allow set `-1` to keep the default value.

### Why are the changes needed?

At Apache Spark 2.2.0, [SPARK-18702](https://github.com/apache/spark/pull/16133/files#diff-2c5519b1cf4308d77d6f12212971544fR27-R38) adds `class FileBlock` with the default `length` value, `-1`, initially.

There is no way to set `filePath` only while keeping `length` is `-1`.
```scala
  def set(filePath: String, startOffset: Long, length: Long): Unit = {
     require(filePath != null, "filePath cannot be null")
     require(startOffset >= 0, s"startOffset ($startOffset) cannot be negative")
     require(length >= 0, s"length ($length) cannot be negative")
     inputBlock.set(new FileBlock(UTF8String.fromString(filePath), startOffset, length))
   }
```

For compressed files (like GZ), the size of split can be set to -1. This was allowed till Spark 2.1 but regressed starting with spark 2.2.x. Please note that split length of -1 also means the length was unknown - a valid scenario. Thus, split length of -1 should be acceptable like pre Spark 2.2.

### Does this PR introduce any user-facing change?

No

### How was this patch tested?

This is updating the corner case on the requirement check. Manually check the code.

Closes #26123 from praneetsharma/fix-SPARK-27259.

Authored-by: prasha2 <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

core/src/main/scala/org/apache/spark/rdd/InputFileBlockHolder.scala

@@ -76,7 +76,7 @@ private[spark] object InputFileBlockHolder {
   def set(filePath: String, startOffset: Long, length: Long): Unit = {
     require(filePath != null, "filePath cannot be null")
     require(startOffset >= 0, s"startOffset ($startOffset) cannot be negative")
-    require(length >= 0, s"length ($length) cannot be negative")
+    require(length >= -1, s"length ($length) cannot be smaller than -1")
     inputBlock.get().set(new FileBlock(UTF8String.fromString(filePath), startOffset, length))
   }