RFR 8213708: Different #ifdef guards cause incorrect use of Monitor::check_block_state()

[Patricio Chilano]

Hi all,

Could you review this small fix to the Monitor class?
Executing method check_block_state() was causing incorrect warnings to 
be printed out in optimized builds due to variable _allow_vm_block being 
incorrectly initialized in the constructor. The fix involved wrapping 
the call to check_block_state() under the DEBUG_ONLY macro.
Built an optimized image with and without the fix, run a couple of tests 
and checked the warning messages appear before the fix but not after it. 
Also built in debug and release modes to check those build okay too.

Since the fix had to do with using the DEBUG_ONLY macro I also changed 
in those files instances of debug_only to DEBUG_ONLY and changed some 
instances of DEBUG_ONLY(code); to DEBUG_ONLY(code;) to make the usage 
consistent.

Webrev URL:
http://cr.openjdk.java.net/~pchilanomate/8213708.01/webrev
Bug URL:
https://bugs.openjdk.java.net/browse/JDK-8213708

Thanks,
Patricio