xfs: fix periodic log flushing

xfs: fix periodic log flushing

[Please take this patch for -stable in kernels 3.5-3.7.  It doesn't have an
equivalent upstream commit because the code was removed before the bug was
discovered.  See f661f1e0bf50 and 7e18530bef6a.]

There is a logic inversion in xfssyncd_worker() which means that the
log is not periodically forced or idled correctly. This means that
metadata changes aggregated in memory do not get flushed in a timely
manner, and hence if filesystem is not cleanly unmounted those
changes can be lost. This loss can manifest itself even hours after
the changes were made if the filesystem is left to idle without a
sync() occurring between the last modification and the
crash/shutdown occuring.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>