kernel: wire I/O accounting into sys_read/sys_write

sys_read increments io_rchar, io_syscr, io_read_bytes.
sys_write increments io_wchar, io_syscw, io_write_bytes.
This makes /proc/[pid]/io show actual I/O activity.

Cross-referenced with Linux 7.1 mm/filemap.c:filemap_get_pages
and include/linux/task_io_accounting.h. The fields are saturated
additions to avoid overflow on long-running processes.

src/syscall/fs.rs

@@ -818,6 +818,14 @@ pub fn sys_read(fd: FileHandle, buf: UserSliceWo, token: &mut CleanLockToken) ->
         let offset = &mut desc_arc.write(token.token()).offset;
         *offset = offset.saturating_add(bytes_read as u64)
     }
+    // I/O accounting: increment rchar and syscr on current context.
+    // Cross-referenced with Linux 7.1 mm/filemap.c:filemap_get_pages
+    // and task_io_account_read().
+    let current = context::current();
+    let mut guard = current.write(token.token());
+    guard.io_rchar = guard.io_rchar.saturating_add(bytes_read as u64);
+    guard.io_syscr = guard.io_syscr.saturating_add(1);
+    guard.io_read_bytes = guard.io_read_bytes.saturating_add(bytes_read as u64);
     Ok(bytes_read)
 }
 pub fn sys_write(fd: FileHandle, buf: UserSliceRo, token: &mut CleanLockToken) -> Result<usize> {
@@ -838,5 +846,11 @@ pub fn sys_write(fd: FileHandle, buf: UserSliceRo, token: &mut CleanLockToken) -
         let offset = &mut desc_arc.write(token.token()).offset;
         *offset = offset.saturating_add(bytes_written as u64)
     }
+    // I/O accounting: increment wchar and syscw on current context.
+    let current = context::current();
+    let mut guard = current.write(token.token());
+    guard.io_wchar = guard.io_wchar.saturating_add(bytes_written as u64);
+    guard.io_syscw = guard.io_syscw.saturating_add(1);
+    guard.io_write_bytes = guard.io_write_bytes.saturating_add(bytes_written as u64);
     Ok(bytes_written)
 }