Documentation: add SCSI drivers' mapping error handling to DMA-API-HOWTO

Add the concrete DMA mapping error handling for SCSI drivers on the
queuecommand path.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/DMA-API-HOWTO.txt

@@ -745,6 +745,10 @@ 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.
 
+SCSI drivers must return SCSI_MLQUEUE_HOST_BUSY if the DMA mapping
+fails in the queuecommand hook. This means that the SCSI subsystem
+passes the command to the driver again later.
+
 			   Closing
 
 This document, and the API itself, would not be in its current