FROMLIST: fuse: Definitions and ioctl() for passthrough

Expose the FUSE_PASSTHROUGH interface to userspace and declare all the
basic data structures and functions as the skeleton on top of which the
FUSE passthrough functionality will be built.

As part of this, introduce the new FUSE passthrough ioctl(), which
allows
the FUSE daemon to specify a direct connection between a FUSE file and a
lower file system file. Such ioctl() requires userspace to pass the file
descriptor of one of its opened files through the fuse_passthrough_out
data
structure introduced in this patch. This structure includes extra fields
for possible future extensions.
Also, add the passthrough functions for the set-up and tear-down of the
data structures and locks that will be used both when fuse_conns and
fuse_files are created/deleted.

Bug: 168023149
Link: https://lore.kernel.org/lkml/20201026125016.1905945-2-balsini@android.com/


Signed-off-by: Alessio Balsini <balsini@android.com>
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: I6dd150b93607e10ed53f7e7975b35b6090080fa2