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

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.

Fixes: 49af6ddb8e5c ("KVM: arm64: Add infrastructure to create and track pKVM instances at EL2")
Reported-by: Ben Simner <ben.simner@cl.cam.ac.uk>
Co-developed-by: Will Deacon <willdeacon@google.com>
Signed-off-by: Will Deacon <willdeacon@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260424084908.370776-6-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org