KVM: arm64: Fix __pkvm_init_vcpu cptr_el2 error path

KVM: arm64: Fix __pkvm_init_vcpu cptr_el2 error path

On an error, hyp_vcpu will be accessed while this memory has already
been relinquished to the host and unmapped from the hypervisor. Protect
the CPTR assignment with an early return.

Fixes: b5b9955617bc ("KVM: arm64: Eagerly restore host fpsimd/sve state in pKVM")
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Link: https://lore.kernel.org/r/20240919110500.2345927-1-vdonnefort@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>