os/FileJournal: fix aio self-throttling deadlock

This block of code tries to limit the number of aios in flight by waiting
for the amount of data to be written to grow relative to a function of the
number of aios.  Strictly speaking, the condition we are waiting for is a
function of both aio_num and the write queue, but we are only woken by
changes in aio_num, and were (in rare cases) waiting when aio_num == 0 and
there was no possibility of being woken.

Fix this by verifying that aio_num > 0, and restructuring the loop to
recheck that condition on each wakeup.

Fixes: #4079
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
osd: reenable 'journal aio = true'

Now that #4079 is resolved.  Reverts 1cfc3ae0.

Signed-off-by: Sage Weil <sage@inktank.com>