FROMLIST: fuse: Introduce synchronous read and write for passthrough

All the read and write operations performed on fuse_files which have the
passthrough feature enabled are forwarded to the associated lower file
system file via VFS.

Sending the request directly to the lower file system avoids the
userspace round-trip that, because of possible context switches and
additional operations might reduce the overall performance, especially
in those cases where caching doesn't help, for example in reads at
random offsets.

Verifying if a fuse_file has a lower file system file associated with
can be done by checking the validity of its passthrough_filp pointer.
This pointer is not NULL only if passthrough has been successfully
enabled via the appropriate ioctl().
When a read/write operation is requested for a FUSE file with
passthrough enabled, a new equivalent VFS request is generated, which
instead targets the lower file system file.
The VFS layer performs additional checks that allow for safer operations
but may cause the operation to fail if the process accessing the FUSE
file system does not have access to the lower file system.

This change only implements synchronous requests in passthrough,
returning an error in the case of asynchronous operations, yet covering
the majority of the use cases.

Bug: 179164095
Link: https://lore.kernel.org/lkml/20210125153057.3623715-6-balsini@android.com/


Signed-off-by: Alessio Balsini <balsini@android.com>
Change-Id: Ifbe6a247fe7338f87d078fde923f0252eeaeb668
Signed-off-by: Alessio Balsini <balsini@google.com>