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/20201026125016.1905945-5-balsini@android.com/


Signed-off-by: Alessio Balsini <balsini@android.com>
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: I1e6b4602e0813c425a41f370c856d2e99e771277