e1000e: fix dma error handling issues

There were a few issues I noticed in e1000e. These include a double free
of the skb if mapping fails, and the fact that context descriptors appear
to be left in the descriptor ring after the failure.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>