uml: define CONFIG_NO_DMA

I think that it's better to detect DMA misuse at build time rather than
calling BUG_ON.  Architectures that can't do DMA need to define
CONFIG_NO_DMA.

Thanks to Sam Ravnborg for explaining how CONFIG_NO_DMA and CONFIG_HAS_DMA
work:

http://marc.info/?l=linux-kernel&m=128359913825550&w=2

HAS_DMA is defined like this:

config HAS_DMA
        boolean
        depends on !NO_DMA
        default y

So to set HAS_DMA to true an arch should do:
1) Do not define NO_DMA
2) Define NO_DMA abd set it to 'n'

Must archs - including um - used principle 1).

In the um case we want to say that we do NOT have any DMA.
This can be done in two ways.
a) define NO_DMA and set it to 'y'
b) redefine HAS_DMA and set it to 'n'.

The patch you provided used principle b) where other archs use principle a).
So I suggest you should use principle a) for um too.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/Kconfig.um

@@ -147,3 +147,6 @@ config KERNEL_STACK_ORDER
 	  This option determines the size of UML kernel stacks.  They will
 	  be 1 << order pages.  The default is OK unless you're running Valgrind
 	  on UML, in which case, set this to 3.
+
+config NO_DMA
+	def_bool y