Documentation: Rename Documentation/DMA-mapping.txt

It seems that Documentation/DMA-mapping.txt was supposed to be renamed
to Documentation/PCI/PCI-DMA-mapping.txt.
Signed-off-by: NKusanagi Kouichi <slash@ac.auone-net.jp>
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/block/biodoc.txt

@@ -186,7 +186,7 @@ a virtual address mapping (unlike the earlier scheme of virtual address
 do not have a corresponding kernel virtual address space mapping) and
 low-memory pages.
 
-Note: Please refer to Documentation/DMA-mapping.txt for a discussion
+Note: Please refer to Documentation/PCI/PCI-DMA-mapping.txt for a discussion
 on PCI high mem DMA aspects and mapping of scatter gather lists, and support
 for 64 bit PCI.