rust: map __kernel_size_t and friends also to usize/isize

Currently bindgen has special logic to recognise `size_t` and `ssize_t`
and map them to Rust `usize` and `isize`. Similarly, `ptrdiff_t` is
mapped to `isize`.

However this falls short for `__kernel_size_t`, `__kernel_ssize_t` and
`__kernel_ptrdiff_t`. To ensure that they are mapped to usize/isize
rather than 32/64 integers depending on platform, blocklist them in
bindgen parameters and manually provide their definition.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Link: https://lore.kernel.org/r/20240913213041.395655-3-gary@garyguo.net
[ Formatted comment. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/uapi/lib.rs

@@ -25,4 +25,9 @@
     unsafe_op_in_unsafe_fn
 )]
 
+// Manual definition of blocklisted types.
+type __kernel_size_t = usize;
+type __kernel_ssize_t = isize;
+type __kernel_ptrdiff_t = isize;
+
 include!(concat!(env!("OBJTREE"), "/rust/uapi/uapi_generated.rs"));