iavf: Fix handling of dummy receive descriptors

In the Linux kernel, the following vulnerability has been resolved: iavf: Fix handling of dummy receive descriptors Fix memory leak caused by not handling dummy receive descriptor properly. iavf_get_rx_buffer now sets the rx_buffer return value for dummy receive descriptors. Without this patch, when the hardware writes a dummy descriptor, iavf would not free the page allocated for the previous receive buffer. This is an unlikely event but can still happen. [Jesse: massaged commit message]