dma-mapping.h: preserve unmap info for CONFIG_DMA_API_DEBUG

When CONFIG_DMA_API_DEBUG is enabled we need to preserve unmapping address
even if "unmap" is a no-op for our architecutre because we need
debug_dma_unmap_page() to correctly cleanup all of the debug bookkeeping.
Failing to do so results in a false positive warnings about previously
mapped areas never being unmapped.

Link: http://lkml.kernel.org/r/1474387125-3713-1-git-send-email-andrew.smirnov@gmail.com
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Zhen Lei <thunder.leizhen@huawei.com>
Cc: "Luis R. Rodriguez" <mcgrof@suse.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Geliang Tang <geliangtang@163.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>