rust: maple_tree: rcu_read_lock() in destructor to silence lockdep

rust: maple_tree: rcu_read_lock() in destructor to silence lockdep

When running the Rust maple tree kunit tests with lockdep, you may trigger
a warning that looks like this:

	lib/maple_tree.c:780 suspicious rcu_dereference_check() usage!

	other info that might help us debug this:

	rcu_scheduler_active = 2, debug_locks = 1
	no locks held by kunit_try_catch/344.

	stack backtrace:
	CPU: 3 UID: 0 PID: 344 Comm: kunit_try_catch Tainted: G                 N  6.19.0-rc1+ #2 NONE
	Tainted: [N]=TEST
	Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
	Call Trace:
	 <TASK>
	 dump_stack_lvl+0x71/0x90
	 lockdep_rcu_suspicious+0x150/0x190
	 mas_start+0x104/0x150
	 mas_find+0x179/0x240
	 _RINvNtCs5QSdWC790r4_4core3ptr13drop_in_placeINtNtCs1cdwasc6FUb_6kernel10maple_tree9MapleTreeINtNtNtBL_5alloc4kbox3BoxlNtNtB1x_9allocator7KmallocEEECsgxAQYCfdR72_25doctests_kernel_generated+0xaf/0x130
	 rust_doctest_kernel_maple_tree_rs_0+0x600/0x6b0
	 ? lock_release+0xeb/0x2a0
	 ? kunit_try_catch_run+0x210/0x210
	 kunit_try_run_case+0x74/0x160
	 ? kunit_try_catch_run+0x210/0x210
	 kunit_generic_run_threadfn_adapter+0x12/0x30
	 kthread+0x21c/0x230
	 ? __do_trace_sched_kthread_stop_ret+0x40/0x40
	 ret_from_fork+0x16c/0x270
	 ? __do_trace_sched_kthread_stop_ret+0x40/0x40
	 ret_from_fork_asm+0x11/0x20
	 </TASK>

This is because the destructor of maple tree calls mas_find() without
taking rcu_read_lock() or the spinlock.  Doing that is actually ok in this
case since the destructor has exclusive access to the entire maple tree,
but it triggers a lockdep warning.  To fix that, take the rcu read lock.

In the future, it's possible that memory reclaim could gain a feature
where it reallocates entries in maple trees even if no user-code is
touching it.  If that feature is added, then this use of rcu read lock
would become load-bearing, so I did not make it conditional on lockdep.

We have to repeatedly take and release rcu because the destructor of T
might perform operations that sleep.

Link: https://lkml.kernel.org/r/20251217-maple-drop-rcu-v1-1-702af063573f@google.com
Fixes: da939ef4c494 ("rust: maple_tree: add MapleTree")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reported-by: Andreas Hindborg <a.hindborg@kernel.org>
Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/x/topic/x/near/564215108
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Cc: Andrew Ballance <andrewjballance@gmail.com>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Trevor Gross <tmgross@umich.edu>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>