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: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>