FROMLIST: fuse: Passthrough initialization and release

Implement the FUSE passthrough ioctl() that associates the lower
(passthrough) file system file with the fuse_file.

The file descriptor passed to the ioctl() by the FUSE daemon is used to
access the relative file pointer, that will be copied to the fuse_file data
structure to consolidate the link between the FUSE and lower file system.

To enable the passthrough mode, userspace triggers the
FUSE_DEV_IOC_PASSTHROUGH_OPEN ioctl() and, if the call succeeds,
receives back an identifier that will be used at open/create response
time in the fuse_open_out field to associate the FUSE file to the lower
file system file.
The value returned by the ioctl() to userspace can be:
- > 0: success, the identifier can be used as part of an open/create
  reply.
- < 0: an error occurred.
The value 0 has been left unused for backward compatibility: the
fuse_open_out field that is used to pass the passthrough_fh back to the
kernel uses the same bits that were previously as struct padding,
zero-initialized in the common libfuse implementation. Removing the 0
value fixes the ambiguity between the case in which 0 corresponds to a
real passthrough_fh or a missing implementation, simplifying the
userspace implementation.

For the passthrough mode to be successfully activated, the lower file
system file must implement both read_ and write_iter file operations.
This extra check avoids special pseudo files to be targeted for this
feature.
Passthrough comes with another limitation: no further file system stacking
is allowed for those FUSE file systems using passthrough.

Bug: 168023149
Link: https://lore.kernel.org/lkml/20201026125016.1905945-3-balsini@android.com/


Signed-off-by: Alessio Balsini <balsini@android.com>
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: Id6067dcb882d58daef5ffe44355c383cabeed547