Documentation: add networking driver's mapping error handling to DMA-API-HOWTO

Adds the concrete DMA mapping error handling for Networking drivers on the
transmit path.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DMA-API-HOWTO.txt

@@ -740,6 +740,11 @@ failure can be determined by:
 		 */
 	}
 
+Networking drivers must call dev_kfree_skb to free the socket buffer
+and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook
+(ndo_start_xmit). This means that the socket buffer is just dropped in
+the failure case.
+
 			   Closing
 
 This document, and the API itself, would not be in its current