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: 443591063
Bug: 395485007
Bug: 357781595
Link: https://lore.kernel.org/all/20250909100007.3136249-5-keirf@google.com/



Tested-by: Li RongQing <lirongqing@baidu.com>
Change-Id: I0ce543e7d72a505884941a76ef5aca093de9da74
Signed-off-by: Keir Fraser <keirf@google.com>