6983311: G1: LoopTest hangs when run with -XX:+ExplicitInvokesConcurrent
Summary: Clear the concurrent marking "in progress" flag while the FullGCCount_lock is held. This avoids a race that can cause back to back System.gc() calls, when ExplicitGCInvokesConcurrent is enabled, to fail to initiate a marking cycle causing the requesting thread to hang.
Reviewed-by: tonyp, ysr