[fix] Too async libata breakage

[Arjan van de Ven]

> > Additional async_synchronize_full() helps and box boot to the end.

Hi Linus,

can you add the patch below to your tree? This makes the
async_synchronize_full() a real absolute async barrier, rather than
just synchronizing the work scheduled upto the starting point.
It's a bit too much for some cases (so later on I might introduce the
softer synchronization again) but it solves some real problems
for people so this should go into -rc1 please...