kmap_atomic: make kunmap_atomic() harder to misuse

kunmap_atomic() is currently at level -4 on Rusty's "Hard To Misuse"
list[1] ("Follow common convention and you'll get it wrong"), except in
some architectures when CONFIG_DEBUG_HIGHMEM is set[2][3].

kunmap() takes a pointer to a struct page; kunmap_atomic(), however, takes
takes a pointer to within the page itself.  This seems to once in a while
trip people up (the convention they are following is the one from
kunmap()).

Make it much harder to misuse, by moving it to level 9 on Rusty's list[4]
("The compiler/linker won't let you get it wrong").  This is done by
refusing to build if the type of its first argument is a pointer to a
struct page.

The real kunmap_atomic() is renamed to kunmap_atomic_notypecheck()
(which is what you would call in case for some strange reason calling it
with a pointer to a struct page is not incorrect in your code).

The previous version of this patch was compile tested on x86-64.

[1] http://ozlabs.org/~rusty/index.cgi/tech/2008-04-01.html
[2] In these cases, it is at level 5, "Do it right or it will always
    break at runtime."
[3] At least mips and powerpc look very similar, and sparc also seems to
    share a common ancestor with both; there seems to be quite some
    degree of copy-and-paste coding here. The include/asm/highmem.h file
    for these three archs mention x86 CPUs at its top.
[4] http://ozlabs.org/~rusty/index.cgi/tech/2008-03-30.html
[5] As an aside, could someone tell me why mn10300 uses unsigned long as
    the first parameter of kunmap_atomic() instead of void *?

Signed-off-by: Cesar Eduardo Barros <[email protected]>
Cc: Russell King <[email protected]> (arch/arm)
Cc: Ralf Baechle <[email protected]> (arch/mips)
Cc: David Howells <[email protected]> (arch/frv, arch/mn10300)
Cc: Koichi Yasutake <[email protected]> (arch/mn10300)
Cc: Kyle McMartin <[email protected]> (arch/parisc)
Cc: Helge Deller <[email protected]> (arch/parisc)
Cc: "James E.J. Bottomley" <[email protected]> (arch/parisc)
Cc: Benjamin Herrenschmidt <[email protected]> (arch/powerpc)
Cc: Paul Mackerras <[email protected]> (arch/powerpc)
Cc: "David S. Miller" <[email protected]> (arch/sparc)
Cc: Thomas Gleixner <[email protected]> (arch/x86)
Cc: Ingo Molnar <[email protected]> (arch/x86)
Cc: "H. Peter Anvin" <[email protected]> (arch/x86)
Cc: Arnd Bergmann <[email protected]> (include/asm-generic)
Cc: Rusty Russell <[email protected]> ("Hard To Misuse" list)
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

arch/mn10300/include/asm/highmem.h

@@ -91,7 +91,7 @@ static inline unsigned long kmap_atomic(struct page *page, enum km_type type)
 	return vaddr;
 }
 
-static inline void kunmap_atomic(unsigned long vaddr, enum km_type type)
+static inline void kunmap_atomic_notypecheck(unsigned long vaddr, enum km_type type)
 {
 #if HIGHMEM_DEBUG
 	enum fixed_addresses idx = type + KM_TYPE_NR * smp_processor_id();