RFR(trivial): 8231024: Improve the debug info when the output is truncated

[Jie Fu]

Hi all,

JBS:    https://bugs.openjdk.java.net/browse/JDK-8231024
Webrev: http://cr.openjdk.java.net/~jiefu/8231024/webrev.00/

*Background*
When I was running a jtreg test with
------------------------------------
make test 
TEST="test/jdk/java/lang/invoke/TestCatchExceptionWithVarargs.java" 
CONF=fast
------------------------------------
I got some VM warnings which just told me to increase O_BUFLEN in 
ostream.hpp.
But the debug info didn't tell us the proper value of O_BUFLEN needed by 
the output.

It might be better to dump the required value of O_BUFLEN.

*Fix*

- Before the patch
------------------------------------
[2019-09-14 16:07:55,752] Agent[1]: stderr: OpenJDK 64-Bit Server VM 
warning: increase O_BUFLEN in ostream.hpp -- output truncated
[2019-09-14 16:07:55,758] Agent[1]: stderr: OpenJDK 64-Bit Server VM 
warning: increase O_BUFLEN in ostream.hpp -- output truncated
[2019-09-14 16:07:55,763] Agent[1]: stderr: OpenJDK 64-Bit Server VM 
warning: increase O_BUFLEN in ostream.hpp -- output truncated
...
------------------------------------

- After the patch
------------------------------------
[2019-09-14 16:19:09,478] Agent[1]: stderr: OpenJDK 64-Bit Server VM 
warning: increase O_BUFLEN in ostream.hpp: at least 2013 is required -- 
output truncated
[2019-09-14 16:19:09,482] Agent[1]: stderr: OpenJDK 64-Bit Server VM 
warning: increase O_BUFLEN in ostream.hpp: at least 2031 is required -- 
output truncated
[2019-09-14 16:19:09,488] Agent[1]: stderr: OpenJDK 64-Bit Server VM 
warning: increase O_BUFLEN in ostream.hpp: at least 2049 is required -- 
output truncated
------------------------------------

Could you please review it?

Thanks a lot.
Best regards,
Jie