mm/slab: use call_rcu() in unknown context if irqs are enabled

call_rcu() disables IRQs with local_irq_save() to protect its per-cpu
data structures. Therefore, if IRQs are not disabled, they cannot be
corrupted by reentrance into call_rcu(). So fall back to the deferred
path only when !allow_spin && irqs_disabled().

The RCU subsystem does not guarantee this contractually, and this
optimization relies on RCU's implementation details. Ideally, it should
be removed once call_rcu_nolock() is supported by the RCU subsystem.

Link: https://lore.kernel.org/linux-mm/CAADnVQKRVD5ZSnEKbZZU7w86gHbGHUug2pvzpgZTngNS+fg4rw@mail.gmail.com
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
Link: https://patch.msgid.link/20260729-kfree_rcu_nolock-v5-3-a28cdcda9673@kernel.org
Reviewed-by: Shengming Hu <hu.shengming@zte.com.cn>
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

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