virtio_vdpa: implement synchronize_cbs()

virtio_vdpa relies on the core's synchronize_rcu() fallback for callback
synchronization. That covers IRQ handlers and sections with interrupts,
preemption or bottom halves disabled. virtio_vdpa does not enforce any
of those contexts when a vDPA driver invokes a callback.

Add an SRCU domain per device and enter it around the virtqueue and
config callbacks. Implement synchronize_cbs() with synchronize_srcu()
so it covers the callbacks regardless of the calling context.

SRCU uses per-CPU reader accounting, avoiding a single callback lock
shared by all queues. Cover the config callback too, as virtio-pci
does for its config vector.

Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260908053817.26065-7-kmehltretter@gmail.com>

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