igb: Initialize mailbox message for VF reset

In the Linux kernel, the following vulnerability has been resolved:

igb: Initialize mailbox message for VF reset

When a MAC address is not assigned to the VF, that portion of the message

sent to the VF is not set. The memory, however, is allocated from the

stack meaning that information may be leaked to the VM. Initialize the

message buffer to 0 so that no information is passed to the VM in this

case.