rust: file: add `Kuid` wrapper

Adds a wrapper around `kuid_t` called `Kuid`. This allows us to define
various operations on kuids such as equality and current_euid. It also
lets us provide conversions from kuid into userspace values.

Rust Binder needs these operations because it needs to compare kuids for
equality, and it needs to tell userspace about the pid and uid of
incoming transactions.

To read kuids from a `struct task_struct`, you must currently use
various #defines that perform the appropriate field access under an RCU
read lock. Currently, we do not have a Rust wrapper for rcu_read_lock,
which means that for this patch, there are two ways forward:

 1. Inline the methods into Rust code, and use __rcu_read_lock directly
    rather than the rcu_read_lock wrapper. This gives up lockdep for
    these usages of RCU.

 2. Wrap the various #defines in helpers and call the helpers from Rust.

This patch uses the second option. One possible disadvantage of the
second option is the possible introduction of speculation gadgets, but
as discussed in [1], the risk appears to be acceptable.

Of course, once a wrapper for rcu_read_lock is available, it is
preferable to use that over either of the two above approaches.

Link: https://lore.kernel.org/all/202312080947.674CD2DC7@keescook/

 [1]
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240915-alice-file-v10-7-88484f7a3dcf@google.com


Signed-off-by: Christian Brauner <brauner@kernel.org>