rust: support srctree-relative links

Some of our links use relative paths in order to point to files in the
source tree, e.g.:

    //! C header: [`include/linux/printk.h`](../../../../include/linux/printk.h)
    /// [`struct mutex`]: ../../../../include/linux/mutex.h

These are problematic because they are hard to maintain and do not support
`O=` builds.

Instead, provide support for `srctree`-relative links, e.g.:

    //! C header: [`include/linux/printk.h`](srctree/include/linux/printk.h)
    /// [`struct mutex`]: srctree/include/linux/mutex.h

The links are fixed after `rustdoc` generation to be based on the absolute
path to the source tree.

Essentially, this is the automatic version of Tomonori's fix [1],
suggested by Gary [2].

Suggested-by: Gary Guo <gary@garyguo.net>
Reported-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Closes: https://lore.kernel.org/r/20231026.204058.2167744626131849993.fujita.tomonori@gmail.com [1]
Fixes: 48fadf4400 ("docs: Move rustdoc output, cross-reference it")
Link: https://lore.kernel.org/rust-for-linux/20231026154525.6d14b495@eugeo/ [2]
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20231215235428.243211-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/sync/condvar.rs

@@ -69,7 +69,7 @@ macro_rules! new_condvar {
 /// }
 /// ```
 ///
-/// [`struct wait_queue_head`]: ../../../include/linux/wait.h
+/// [`struct wait_queue_head`]: srctree/include/linux/wait.h
 #[pin_data]
 pub struct CondVar {
     #[pin]

rust/kernel/sync/lock/mutex.rs

@@ -84,7 +84,7 @@ macro_rules! new_mutex {
 /// }
 /// ```
 ///
-/// [`struct mutex`]: ../../../../include/linux/mutex.h
+/// [`struct mutex`]: srctree/include/linux/mutex.h
 pub type Mutex<T> = super::Lock<T, MutexBackend>;
 
 /// A kernel `struct mutex` lock backend.

rust/kernel/sync/lock/spinlock.rs

@@ -82,7 +82,7 @@ macro_rules! new_spinlock {
 /// }
 /// ```
 ///
-/// [`spinlock_t`]: ../../../../include/linux/spinlock.h
+/// [`spinlock_t`]: srctree/include/linux/spinlock.h
 pub type SpinLock<T> = super::Lock<T, SpinLockBackend>;
 
 /// A kernel `spinlock_t` lock backend.