rust: transmute: simplify code with Rust 1.80.0 split_at_*checked()

`feature(split_at_checked)` [1] has been stabilized in Rust 1.80.0 [2],
which is older than our new minimum Rust version (Rust 1.85.0).

Thus simplify the code using `split_at_*checked()`.

Link: rust-lang/rust#119128 [1]
Link: rust-lang/rust#124678 [2]
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-14-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/transmute.rs

@@ -66,16 +66,9 @@ pub unsafe trait FromBytes {
     where
         Self: Sized,
     {
-        if bytes.len() < size_of::<Self>() {
-            None
-        } else {
-            // PANIC: We checked that `bytes.len() >= size_of::<Self>`, thus `split_at` cannot
-            // panic.
-            // TODO: replace with `split_at_checked` once the MSRV is >= 1.80.
-            let (prefix, remainder) = bytes.split_at(size_of::<Self>());
+        let (prefix, remainder) = bytes.split_at_checked(size_of::<Self>())?;
 
-            Self::from_bytes(prefix).map(|s| (s, remainder))
-        }
+        Self::from_bytes(prefix).map(|s| (s, remainder))
     }
 
     /// Converts a mutable slice of bytes to a reference to `Self`.
@@ -108,16 +101,9 @@ pub unsafe trait FromBytes {
     where
         Self: AsBytes + Sized,
     {
-        if bytes.len() < size_of::<Self>() {
-            None
-        } else {
-            // PANIC: We checked that `bytes.len() >= size_of::<Self>`, thus `split_at_mut` cannot
-            // panic.
-            // TODO: replace with `split_at_mut_checked` once the MSRV is >= 1.80.
-            let (prefix, remainder) = bytes.split_at_mut(size_of::<Self>());
+        let (prefix, remainder) = bytes.split_at_mut_checked(size_of::<Self>())?;
 
-            Self::from_bytes_mut(prefix).map(|s| (s, remainder))
-        }
+        Self::from_bytes_mut(prefix).map(|s| (s, remainder))
     }
 
     /// Creates an owned instance of `Self` by copying `bytes`.
@@ -147,16 +133,9 @@ pub unsafe trait FromBytes {
     where
         Self: Sized,
     {
-        if bytes.len() < size_of::<Self>() {
-            None
-        } else {
-            // PANIC: We checked that `bytes.len() >= size_of::<Self>`, thus `split_at` cannot
-            // panic.
-            // TODO: replace with `split_at_checked` once the MSRV is >= 1.80.
-            let (prefix, remainder) = bytes.split_at(size_of::<Self>());
+        let (prefix, remainder) = bytes.split_at_checked(size_of::<Self>())?;
 
-            Self::from_bytes_copy(prefix).map(|s| (s, remainder))
-        }
+        Self::from_bytes_copy(prefix).map(|s| (s, remainder))
     }
 }