xfs: re-enable xfsaild idle mode and fix associated races

xfsaild idle mode logic currently leads to a couple hangs:

1.) If xfsaild is rescheduled in during an incremental scan
    (i.e., tout != 0) and the target has been updated since
    the previous run, we can hit the new target and go into
    idle mode with a still populated ail.
2.) A wake up is only issued when the target is pushed forward.
    The wake up can race with xfsaild if it is currently in the
    process of entering idle mode, causing future wake up
    events to be lost.

These hangs have been reproduced and verified as fixed by
running xfstests 273 in a loop on a slightly modified upstream
kernel. The kernel is modified to re-enable idle mode as
previously implemented (when count == 0) and with a revert of
commit 670ce93f, which includes performance improvements that
make this harder to reproduce.

The solution, the algorithm for which has been outlined by
Dave Chinner, is to modify xfsaild to enter idle mode only when
the ail is empty and the push target has not been moved forward
since the last push.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>