virtio-net: make all RX paths handle errors consistently

receive mergeable now handles errors internally.
Do same for big and small packet paths, otherwise
the logic is too hard to follow.

Cc: Jason Wang <jasowang@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Acked-by: Michael Dalton <mwdalton@google.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit f121159d72091f25afb22007c833e60a6845e912)
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>