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: 168023149
Link: https://lore.kernel.org/lkml/20201026125016.1905945-4-balsini@android.com/


Signed-off-by: Alessio Balsini <balsini@android.com>
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: If76bb8725e1ac567f9dbe3edb79ebb4d43d77dfb