KVM: nVMX: Assert that vcpu->mutex is held when accessing secondary VMCSes

Add lockdep assertions in get_vmcs12() and get_shadow_vmcs12() to verify
the vCPU's mutex is held, as the returned VMCS objects are dynamically
allocated/freed when nested VMX is turned on/off, i.e. accessing vmcs12
structures without holding vcpu->mutex is susceptible to use-after-free.

Waive the assertion if the VM is being destroyed, as KVM currently forces
a nested VM-Exit when freeing the vCPU.  If/when that wart is fixed, the
assertion can/should be converted to an unqualified lockdep assertion.
See also https://lore.kernel.org/all/Zsd0TqCeY3B5Sb5b@google.com.

Link: https://lore.kernel.org/r/20240906043413.1049633-8-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>