file-posix: Move read/write operation logic out of aio_worker()

file-posix: Move read/write operation logic out of aio_worker()

aio_worker() for reads and writes isn't boring enough yet. It still does
some postprocessing for handling short reads and turning the result into
the right return value.

However, there is no reason why handle_aiocb_rw() couldn't do the same,
and even without duplicating code between the read and write path. So
move the code there.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>