JFS: Don't set log_SYNCBARRIER when log->active == 0

If a metadata page is kept active, it is possible that the sync barrier logic
continues to trigger, even if all active transactions have been phyically
written to the journal.  This can cause a hang, since the completion of the
journal I/O is what unsets the sync barrier flag to allow new transactions
to be created.

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>