octeon_ep: Add SKB allocation failures handling in __octep_oq_process_rx()

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.

Fixes: 37d79d059606 ("octeon_ep: add Tx/Rx processing and interrupt support")
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>