KVM: arm64: Check arm64_get_bp_hardening_data() didn't return NULL

KVM: arm64: Check arm64_get_bp_hardening_data() didn't return NULL

BugLink: https://bugs.launchpad.net/bugs/1971497
Will reports that with CONFIG_EXPERT=y and CONFIG_HARDEN_BRANCH_PREDICTOR=n,
the kernel dereferences a NULL pointer during boot:

[    2.384444] Internal error: Oops: 96000004 [#1] PREEMPT SMP
[    2.384461] pstate: 20400085 (nzCv daIf +PAN -UAO)
[    2.384472] pc : cpu_hyp_reinit+0x114/0x30c
[    2.384476] lr : cpu_hyp_reinit+0x80/0x30c

[    2.384529] Call trace:
[    2.384533]  cpu_hyp_reinit+0x114/0x30c
[    2.384537]  _kvm_arch_hardware_enable+0x30/0x54
[    2.384541]  flush_smp_call_function_queue+0xe4/0x154
[    2.384544]  generic_smp_call_function_single_interrupt+0x10/0x18
[    2.384549]  ipi_handler+0x170/0x2b0
[    2.384555]  handle_percpu_devid_fasteoi_ipi+0x120/0x1cc
[    2.384560]  __handle_domain_irq+0x9c/0xf4
[    2.384563]  gic_handle_irq+0x6c/0xe4
[    2.384566]  el1_irq+0xf0/0x1c0
[    2.384570]  arch_cpu_idle+0x28/0x44
[    2.384574]  do_idle+0x100/0x2a8
[    2.384577]  cpu_startup_entry+0x20/0x24
[    2.384581]  secondary_start_kernel+0x1b0/0x1cc
[    2.384589] Code: b9469d08 7100011f 540003ad 52800208 (f9400108)
[    2.384600] ---[ end trace 266d08dbf96ff143 ]---
[    2.385171] Kernel panic - not syncing: Fatal exception in interrupt

In this configuration arm64_get_bp_hardening_data() returns NULL.
Add a check in kvm_get_hyp_vector().

Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/linux-arm-kernel/20220408120041.GB27685@willie-the-truck/
Fixes: 26129ea2953b ("KVM: arm64: Add templates for BHB mitigation sequences")
Cc: stable@vger.kernel.org # 5.4.x
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>