arch: reevaluate #[allow] attributes

Remove stale #[allow]s whose lints no longer fire (a module-level
non_camel_case_types in mpspec, and a too_many_arguments on a riscv64
configure_system that no longer exceeds the argument threshold), and
convert the still-needed ones to #[expect].

Part of #8326.

Signed-off-by: Tushar Khatri <hello@tusharkhatri.in>

arch/src/riscv64/fdt.rs

@@ -61,7 +61,7 @@ pub enum Error {
 type Result<T> = result::Result<T, Error>;
 
 /// Creates the flattened device tree for this riscv64 VM.
-#[allow(clippy::too_many_arguments)]
+#[expect(clippy::too_many_arguments)]
 pub fn create_fdt<T: DeviceInfoForFdt + Clone + Debug, S: ::std::hash::BuildHasher>(
     guest_mem: &GuestMemoryMmap,
     cmdline: &str,

arch/src/riscv64/mod.rs

@@ -160,7 +160,6 @@ fn isa_string_from_host() -> Result<String, Error> {
 }
 
 /// Configures the system and should be called once per vm before starting vcpu threads.
-#[allow(clippy::too_many_arguments)]
 pub fn configure_system<T: DeviceInfoForFdt + Clone + Debug, S: ::std::hash::BuildHasher>(
     guest_mem: &GuestMemoryMmap,
     cmdline: &str,