rust: task: restrict Task::group_leader() to current

The Task::group_leader() method currently allows you to access the
group_leader() of any task, for example one you hold a refcount to.  But
this is not safe in general since the group leader could change when a
task exits.  See for example commit a15f37a40145c ("kernel/sys.c: fix the
racy usage of task_lock(tsk->group_leader) in sys_prlimit64() paths").

All existing users of Task::group_leader() call this method on current,
which is guaranteed running, so there's not an actual issue in Rust code
today.  But to prevent code in the future from making this mistake,
restrict Task::group_leader() so that it can only be called on current.

There are some other cases where accessing task->group_leader is okay. 
For example it can be safe if you hold tasklist_lock or rcu_read_lock(). 
However, only supporting current->group_leader is sufficient for all
in-tree Rust users of group_leader right now.  Safe Rust functionality for
accessing it under rcu or while holding tasklist_lock may be added in the
future if required by any future Rust module.

This patch is a bugfix in that it prevents users of this API from writing
incorrect code.  It doesn't change behavior of correct code.

Link: https://lkml.kernel.org/r/20260107-task-group-leader-v2-1-8fbf816f2a2f@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Fixes: 313c4281bc9d ("rust: add basic `Task`")
Reported-by: Oleg Nesterov <oleg@redhat.com>
Closes: https://lore.kernel.org/all/aTLnV-5jlgfk1aRK@redhat.com/
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Benno Lossin <lossin@kernel.org>
Cc: "Björn Roy Baron" <bjorn3_gh@protonmail.com>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: FUJITA Tomonori <fujita.tomonori@gmail.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Panagiotis Foliadis <pfoliadis@posteo.net>
Cc: Shankari Anand <shankari.ak0208@gmail.com>
Cc: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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