ANDROID: fuse/passthrough: API V2 with __u32 open argument

The initial FUSE passthrough interface has the issue of introducing an
ioctl which receives as a parameter a data structure containing a
pointer. What happens is that, depending on the architecture, the size
of this struct might change, and especially for 32-bit userspace running
on 64-bit kernel, the size mismatch results into different a single
ioctl the behavior of which depends on the data that is passed (e.g.,
with an enum). This is just a poor ioctl design as mentioned by Arnd
Bergmann [1].

Introduce the new FUSE_PASSTHROUGH_OPEN ioctl which only gets the fd of
the lower file system, which is a fixed-size __u32, dropping the
confusing fuse_passthrough_out data structure.

[1] https://lore.kernel.org/lkml/CAK8P3a2K2FzPvqBYL9W=Yut58SFXyetXwU4Fz50G5O3TsS0pPQ@mail.gmail.com/



Bug: 175195837
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: I486d71cbe20f3c0c87544fa75da4e2704fe57c7c