KVM: arm64: Fix pin leak and publication ordering in __pkvm_init_vcpu()

In the Linux kernel, the following vulnerability has been resolved:

KVM: arm64: Fix pin leak and publication ordering in __pkvm_init_vcpu()

Two bugs exist in the vCPU initialisation path:

1. If a check fails after hyp_pin_shared_mem() succeeds, the cleanup

path jumps to 'unlock' without calling unpin_host_vcpu() or

unpin_host_sve_state(), permanently leaking pin references on the

host vCPU and SVE state pages.

Extract a register_hyp_vcpu() helper that performs the checks and

the store. When register_hyp_vcpu() returns an error, call

unpin_host_vcpu() and unpin_host_sve_state() inline before falling

through to the existing 'unlock' label.

2. register_hyp_vcpu() publishes the new vCPU pointer into

'hyp_vm->vcpus[]' with a bare store, allowing a concurrent caller

of pkvm_load_hyp_vcpu() to observe a partially initialised vCPU

object.

Ensure the store uses smp_store_release() and the load uses

smp_load_acquire(). While 'vm_table_lock' currently serialises the

store and the load, these barriers ensure the reader sees the fully

initialised 'hyp_vcpu' object even if there were a lockless path or

if the lock's own ordering guarantees were insufficient for nested

object initialization.