x86/tracing: comment need for atomic nop

The dynamic function tracer relys on the macro P6_NOP5 always being
an atomic NOP. If for some reason it is changed to be two operations
(like a nop2 nop3) it can faults within the kernel when the function
tracer modifies the code.

This patch adds a comment to note that the P6_NOPs are expected to
be atomic. This will hopefully prevent anyone from changing that.
Reported-by: Mathieu Desnoyer <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

arch/x86/include/asm/nops.h

@@ -65,6 +65,8 @@
    6: osp nopl 0x00(%eax,%eax,1)
    7: nopl 0x00000000(%eax)
    8: nopl 0x00000000(%eax,%eax,1)
+   Note: All the above are assumed to be a single instruction.
+	There is kernel code that depends on this.
 */
 #define P6_NOP1	GENERIC_NOP1
 #define P6_NOP2	".byte 0x66,0x90\n"