dma-mapping: move all DMA mapping code to kernel/dma

Currently the code is split over various files with dma- prefixes in the
lib/ and drives/base directories, and the number of files keeps growing.
Move them into a single directory to keep the code together and remove
the file name prefixes.  To match the irq infrastructure this directory
is placed under the kernel/ directory.

Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/include/linux/dma-contiguous.h b/include/linux/dma-contiguous.h
index b67bf6a..3c5a4cb 100644
--- a/include/linux/dma-contiguous.h
+++ b/include/linux/dma-contiguous.h
@@ -48,7 +48,7 @@
  *   CMA should not be used by the device drivers directly. It is
  *   only a helper framework for dma-mapping subsystem.
  *
- *   For more information, see kernel-docs in drivers/base/dma-contiguous.c
+ *   For more information, see kernel-docs in kernel/dma/contiguous.c
  */
 
 #ifdef __KERNEL__