Correctly set capacity of remainder in DefaultDataBuffer::split

This commit ensures that the capacity of the remainder buffer after a
split operation is set directly on the field. Calling capacity(int)
caused a new buffer to be allocated.

See gh-31848
Closes gh-31859

spring-core/src/main/java/org/springframework/core/io/buffer/DefaultDataBuffer.java

@@ -371,7 +371,7 @@ public DataBuffer split(int index) {
 				.slice();
 		this.writePosition = Math.max(this.writePosition, index) - index;
 		this.readPosition = Math.max(this.readPosition, index) - index;
-		capacity(this.byteBuffer.capacity());
+		this.capacity = this.byteBuffer.capacity();
 
 		return result;
 	}