x86_64: Remove CLFLUSH in text_poke()

The CLFLUSH for the modified code line in text_poke was supposed
to speed up CPU recovery. Unfortunately it seems to cause hangs
on some VIA C3s (at least on VIA Esther Model 10 Stepping 9)
Remove it.

Thanks to Stefan Becker for reporting/testing.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>