LU-13805 llite: wait for partially successful aio

LU-13805 llite: wait for partially successful aio

For various reasons (notably conflicting buffered IO), we
may need to fall back from DIO to buffered IO.  This also
affects AIO, and if it happens, we will sometimes submit
only part of an AIO with the AIO path, completing the rest
with the buffered path.

Userspace doesn't expect this, expecting us to either do
all or none of the IO with AIO, so it doesn't wait for
completion in this case.

To meet this expectation, we must recognize this case and
wait for AIO to complete before returning to userspace.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Iac7abac3bd01f027c353120483932a62c6475277