rust: cleanup unnecessary casts

With `long` mapped to `isize`, `size_t`/`__kernel_size_t` mapped to
`usize` and `char` mapped to `u8`, many of the existing casts are no
longer necessary.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240913213041.395655-6-gary@garyguo.net


[ Moved `uaccess` changes to the previous commit, since they were
  irrefutable patterns that Rust >= 1.82.0 warns about. Removed a
  couple casts that now use `c""` literals. Rebased on top of
  `rust-next`. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>