x86: Clarify/fix no-op barriers for text_poke_bp()

So I was looking at text_poke_bp() today and I couldn't make sense of
the barriers there.

How's for something like so?

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: masami.hiramatsu.pt@hitachi.com
Link: http://lkml.kernel.org/r/20170731102154.f57cvkjtnbmtctk6@hirez.programming.kicks-ass.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>