xfs: revert AIL TASK_KILLABLE threshold

In commit 9adf4024, we changed the behavior of the AIL thread to
set its own task state to KILLABLE whenever the timeout value is
nonzero.  Unfortunately, this missed the fact that xfsaild_push will
return 50ms (aka a longish sleep) when we reach the push target or the
AIL becomes empty, so xfsaild goes to sleep for a long period of time in
uninterruptible D state.

This results in artificially high load averages because KILLABLE
processes are UNINTERRUPTIBLE, which contributes to load average even
though the AIL is asleep waiting for someone to interrupt it.  It's not
blocked on IOs or anything, but people scrap ps for processes that look
like they're stuck in D state, so restore the previous threshold.

Fixes: 9adf4024 ("xfs: AIL doesn't need manual pushing")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>