irqchip/sifive-plic: Fixup EOI failed when masked

When using "devm_request_threaded_irq(,,,,IRQF_ONESHOT,,)" in a driver,
only the first interrupt is handled, and following interrupts are never
delivered (initially reported in [1]).

That's because the RISC-V PLIC cannot EOI masked interrupts, as explained
in the description of Interrupt Completion in the PLIC spec [2]:

<quote>
The PLIC signals it has completed executing an interrupt handler by
writing the interrupt ID it received from the claim to the claim/complete
register. The PLIC does not check whether the completion ID is the same
as the last claim ID for that target. If the completion ID does not match
an interrupt source that *is currently enabled* for the target, the
completion is silently ignored.
</quote>

Re-enable the interrupt before completion if it has been masked during
the handling, and remask it afterwards.

[1] http://lists.infradead.org/pipermail/linux-riscv/2021-July/007441.html
[2] https://github.com/riscv/riscv-plic-spec/blob/8bc15a35d07c9edf7b5d23fec9728302595ffc4d/riscv-plic.adoc

Fixes: bb0fed1c60cc ("irqchip/sifive-plic: Switch to fasteoi flow")
Reported-by: Vincent Pelletier <plr.vincent@gmail.com>
Tested-by: Nikita Shubin <nikita.shubin@maquefel.me>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: stable@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
[maz: amended commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211105094748.3894453-1-guoren@kernel.org

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