FROMLIST: KVM: Avoid synchronize_srcu() in kvm_io_bus_register_dev()

Device MMIO registration may happen quite frequently during VM boot,
and the SRCU synchronization each time has a measurable effect
on VM startup time. In our experiments it can account for around 25%
of a VM's startup time.

Replace the synchronization with a deferred free of the old kvm_io_bus
structure.

Bug: 395485007
Bug: 357781595
Link: https://lore.kernel.org/all/20250624092256.1105524-4-keirf@google.com/



Change-Id: I52af8db54259f4423e3dcdfa298573944796d88d
Signed-off-by: Keir Fraser <keirf@google.com>