x86, atomic: atomic64_read() take a const pointer

atomic64_read() doesn't actually write anything (as far as the C
environment is concerned... the CPU does actually write but that's an
implementation quirk), so it should take a const pointer.

This does NOT mean that it is safe to use atomic64_read() on an object
in readonly storage (it will trap!)
Reported-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/20120109165859.1879abda.akpm@linux-foundation.org

arch/x86/include/asm/atomic64_32.h

@@ -82,7 +82,7 @@ static inline void atomic64_set(atomic64_t *v, long long i)
  *
  * Atomically reads the value of @v and returns it.
  */
-static inline long long atomic64_read(atomic64_t *v)
+static inline long long atomic64_read(const atomic64_t *v)
 {
 	long long r;
 	asm volatile(ATOMIC64_ALTERNATIVE(read)