KVM: Fix a goof where kvm_create_vm() returns 0 instead of -ENOMEM

KVM: Fix a goof where kvm_create_vm() returns 0 instead of -ENOMEM

The error path for OOM when allocating buses used to return -ENOMEM using
the local variable 'r', where 'r' was initialized at the top of the
function.  But a new "r = kvm_init_irq_routing(kvm);" was introduced in
the middle of the function, so now the error code is not set and it
eventually leads to a NULL dereference due to kvm_dev_ioctl_create_vm()
thinking kvm_create_vm() succeeded.  Set the error code back to -ENOMEM.

Opportunistically tweak the logic to pre-set "r = -ENOMEM" immediately
before the flows that can fail due to memory allocation failure to make
it less likely that the bug recurs in the future.

Fixes: fbe4a7e881d4 ("KVM: Setup empty IRQ routing when creating a VM")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/02051e0a-09d8-49a2-917f-7c2f278a1ba1@moroto.mountain
[sean: tweak all of the "r = -ENOMEM" sites, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>