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: Kusanagi Kouichi <[e-mail]>
Signed-off-by: Randy Dunlap <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/block/biodoc.txt

···
 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.