`rustdoc`: Clean `private_intra_doc_links` warnings and make them `deny`

[ojeda]

warning: public documentation for `File` links to private item `File::ptr`
  --> rust/kernel/file.rs:15:18
   |
15 | /// The pointer [`File::ptr`] is non-null and valid. Its reference count is also non-zero.
   |                  ^^^^^^^^^^^ this item is private
   |
   = note: `#[warn(rustdoc::private_intra_doc_links)]` on by default
   = note: this link will resolve properly if you pass `--document-private-items`

warning: public documentation for `PollTable` links to private item `PollTable::ptr`
  --> rust/kernel/file_operations.rs:28:18
   |
28 | /// The pointer [`PollTable::ptr`] is null or valid.
   |                  ^^^^^^^^^^^^^^^^ this item is private
   |
   = note: this link will resolve properly if you pass `--document-private-items`

warning: public documentation for `Pages` links to private item `Pages::pages`
  --> rust/kernel/pages.rs:34:18
   |
34 | /// The pointer [`Pages::pages`] is valid and points to 2^ORDER pages.
   |                  ^^^^^^^^^^^^^^ this item is private
   |
   = note: this link will resolve properly if you pass `--document-private-items`

warning: public documentation for `IovIter` links to private item `IovIter::ptr`
  --> rust/kernel/iov_iter.rs:32:18
   |
32 | /// The pointer [`IovIter::ptr`] is non-null and valid.
   |                  ^^^^^^^^^^^^^^ this item is private
   |
   = note: this link will resolve properly if you pass `--document-private-items`

warning: 4 warnings emitted

[nbdd0121]

I think we should warn these warnings (including missing_docs) locally and deny them in the CI.

[ojeda]

This was done in #412.