octeon_ep: Add SKB allocation failures handling in __octep_oq_process_rx()

In the Linux kernel, the following vulnerability has been resolved: octeon_ep: Add SKB allocation failures handling in __octep_oq_process_rx() build_skb() returns NULL in case of a memory allocation failure so handle it inside __octep_oq_process_rx() to avoid NULL pointer dereference. __octep_oq_process_rx() is called during NAPI polling by the driver. If skb allocation fails, keep on pulling packets out of the Rx DMA queue: we shouldn't break the polling immediately and thus falsely indicate to the octep_napi_poll() that the Rx pressure is going down. As there is no associated skb in this case, don't process the packets and don't push them up the network stack - they are skipped. Helper function is implemented to unmmap/flush all the fragment buffers used by the dropped packet. 'alloc_failures' counter is incremented to mark the skb allocation error in driver statistics. Found by Linux Verification Center (linuxtesting.org) with SVACE.