KVM: selftests: Zero out valid_bank_mask for "all" case in Hyper-V IPI test

Zero out the valid_bank_mask when using the fast variant of
HVCALL_SEND_IPI_EX to send IPIs to all vCPUs.  KVM requires the "var_cnt"
and "valid_bank_mask" inputs to be consistent even when targeting all
vCPUs.  See commit bd1ba573 ("KVM: x86: Get the number of Hyper-V
sparse banks from the VARHEAD field").

Fixes: 99848924 ("KVM: selftests: Hyper-V PV IPI selftest")
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221219220416.395329-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>