rust: miscdevice: add base miscdevice abstraction

rust: miscdevice: add base miscdevice abstraction

Provide a `MiscDevice` trait that lets you specify the file operations
that you wish to provide for your misc device. For now, only three file
operations are provided: open, close, ioctl.

These abstractions only support MISC_DYNAMIC_MINOR. This enforces that
new miscdevices should not hard-code a minor number.

When implementing ioctl, the Result type is used. This means that you
can choose to return either of:
* An integer of type isize.
* An errno using the kernel::error::Error type.
When returning an isize, the integer is returned verbatim. It's mainly
intended for returning positive integers to userspace. However, it is
technically possible to return errors via the isize return value too.

To avoid having a dependency on files, this patch does not provide the
file operations callbacks a pointer to the file. This means that they
cannot check file properties such as O_NONBLOCK (which Binder needs).
Support for that can be added as a follow-up.

To avoid having a dependency on vma, this patch does not provide any way
to implement mmap (which Binder needs). Support for that can be added as
a follow-up.

Rust Binder will use these abstractions to create the /dev/binder file
when binderfs is disabled.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/rust-for-linux/20240328195457.225001-1-wedsonaf@gmail.com/
Link: https://lore.kernel.org/r/20241001-b4-miscdevice-v2-2-330d760041fa@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>