In txLazyCommit, we are releasing log->gclock (LOGGC_LOCK) before
checking tblk->flag for tblkGC_LAZY.  For the case that tblkGC_LAZY
is not set, the user thread may release the tblk, and it may be
reused and the tblkGC_LAZY bit set again, between the time we release
the spinlock until we check the flag.  This is a lot to happen in an
SMP environment, but when CONFIG_PREEMPT is set, it is very easy to
see the problem.

The fix is to hold the spinlock until after we've checked the flag.

(Yes, I know the symbol names are ugly.)