8041468: Field nmethod::_lock_count should be declared volatile
Summary: The jint field nmethod::_lock_count which is used in nmethodLocker::lock_nmethod and nmethodLocker::unlock_nmethod should be declared volatile (see also signature of Atomic::inc)
Reviewed-by: kvn, roland
Contributed-by: Tobias Hartmann <tobias.hartmann@oracle.com>