FROMLIST: fuse: Handle asynchronous read and write in passthrough

Extend the passthrough feature by handling asynchronous IO both for read
and write operations.

When an AIO request is received, if the request targets a FUSE file with
the passthrough functionality enabled, a new identical AIO request is
created. The new request targets the lower file system file and gets
assigned a special FUSE passthrough AIO completion callback.
When the lower file system AIO request is completed, the FUSE
passthrough AIO completion callback is executed and propagates the
completion signal to the FUSE AIO request by triggering its completion
callback as well.

Bug: 168023149
Link: https://lore.kernel.org/lkml/20210125153057.3623715-7-balsini@android.com/


Signed-off-by: Alessio Balsini <balsini@android.com>
Change-Id: I47671ef36211102da6dd3ee8b2f226d1e6cd9d5c
Signed-off-by: Alessio Balsini <balsini@google.com>