mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT

As suggested by Vlastimil Babka [1], kfree_rcu_sheaf() can be used
on PREEMPT_RT if we always assume spinning is not allowed on PREEMPT_RT.
This is because local_trylock and spinlock_t are safe to use with
trylock and unlock as long as the kernel does not spin and the context
is not NMI and not hardirq.

Now that __kfree_rcu_sheaf() knows how to handle SLAB_FREE_NOLOCK,
relax the limitation and try the sheaves path on PREEMPT_RT as well.

Keep the lockdep map on non RT kernels. However, do not use the lockdep
map on PREEMPT_RT to avoid suppressing valid lockdep warnings.

As pointed by Vlastimil Babka [2], on PREEMPT_RT it is unnecessary to
defer call_rcu() under IRQ-disabled section or raw spinlock. However,
let us avoid adding more complexity as the scenario is not supposed
to be common on PREEMPT_RT, with a hope that call_rcu_nolock() will be
soon supported in RCU.

Link: https://lore.kernel.org/linux-mm/6811cc17-8ee4-48c8-8cbf-6bf4d9f98162@kernel.org [1]
Link: https://lore.kernel.org/linux-mm/40591888-3a87-433e-b3d2-cda1cab543be@kernel.org [2]
Suggested-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260729-kfree_rcu_nolock-v5-5-a28cdcda9673@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

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