[RFR] 8235857: Add the parameter cushion length to WriterHost::ensure_size

[Denghui Dong]

Hi team:

Please help review the following changeset:

Bug: https://bugs.openjdk.java.net/browse/JDK-8235857
Webrev: http://cr.openjdk.java.net/~ddong/8235857/webrev.00/

Summary:
There is a path to call WriterHost::write(const T* value, size_t len) in WriterHost::write(jstring string),
it's necessary to pass the length of jchar so that WriterHost::ensure_size can know the length of cushion.
For the java event side, the max length of the cushion has already been counted in the method "jdk.jfr.internal.EventWriter#putStringValue".

Thanks,
Denghui Dong