atomic: Update comments in atomic.h

This clarifies the differences between <linux/atomic.h> and
<asm-generic/atomic.h>
Signed-off-by: NArun Sharma <asharma@fb.com>
Suggested-by: NMike Frysinger <vapier.adi@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: David Miller <davem@davemloft.net>
Acked-by: NMike Frysinger <vapier@gentoo.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/atomic.h

 /*
- * Generic C implementation of atomic counter operations
+ * Generic C implementation of atomic counter operations. Usable on
+ * UP systems only. Do not include in machine independent code.
+ *
  * Originally implemented for MN10300.
  *
  * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.

include/linux/atomic.h

+/* Atomic operations usable in machine independent code */
 #ifndef _LINUX_ATOMIC_H
 #define _LINUX_ATOMIC_H
 #include <asm/atomic.h>