irqchip/gic-v3: Force propagation of the active state with a read-back

Christoffer reports that on some implementations, writing to
GICR_ISACTIVER0 (and similar GICD registers) can race badly with a guest
issuing a deactivation of that interrupt via the system register interface.

There are multiple reasons to this:

 - this uses an early write-acknoledgement memory type (nGnRE), meaning
   that the write may only have made it as far as some interconnect
   by the time the store is considered "done"

 - the GIC itself is allowed to buffer the write until it decides to
   take it into account (as long as it is in finite time)

The effects are that the activation may not have taken effect by the time
the kernel enters the guest, forcing an immediate exit, or that a guest
deactivation occurs before the interrupt is active, doing nothing.

In order to guarantee that the write to the ISACTIVER register has taken
effect, read back from it, forcing the interconnect to propagate the write,
and the GIC to process the write before returning the read.

Reported-by: Christoffer Dall <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Christoffer Dall <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/all/[email protected]

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