rust: uaccess: add UserSliceReader::read_slice_file()

Add UserSliceReader::read_slice_file(), which is the same as
UserSliceReader::read_slice_partial() but updates the given file::Offset
by the number of bytes read.

This is equivalent to C's `simple_write_to_buffer()` and useful when
dealing with file offsets from file operations.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
[ Replace saturating_add() with the raw operator and a corresponding
  OVERFLOW comment. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/uaccess.rs

@@ -9,6 +9,7 @@ use crate::{
     bindings,
     error::Result,
     ffi::{c_char, c_void},
+    fs::file,
     prelude::*,
     transmute::{AsBytes, FromBytes},
 };
@@ -304,6 +305,31 @@ impl UserSliceReader {
         Ok(dst.len())
     }
 
+    /// Reads raw data from the user slice into a kernel buffer partially.
+    ///
+    /// This is the same as [`Self::read_slice_partial`] but updates the given [`file::Offset`] by
+    /// the number of bytes read.
+    ///
+    /// This is equivalent to C's `simple_write_to_buffer()`.
+    ///
+    /// On success, returns the number of bytes read.
+    pub fn read_slice_file(&mut self, out: &mut [u8], offset: &mut file::Offset) -> Result<usize> {
+        if offset.is_negative() {
+            return Err(EINVAL);
+        }
+
+        let Ok(offset_index) = (*offset).try_into() else {
+            return Ok(0);
+        };
+
+        let read = self.read_slice_partial(out, offset_index)?;
+
+        // OVERFLOW: `offset + read <= data.len() <= isize::MAX <= Offset::MAX`
+        *offset += read as i64;
+
+        Ok(read)
+    }
+
     /// Reads a value of the specified type.
     ///
     /// Fails with [`EFAULT`] if the read happens on a bad address, or if the read goes out of