powerpc: ftrace, added missing icache flush

Impact: fix to PowerPC code modification

After modifying code it is essential to flush the icache. This patch
adds the missing flush.

Reported-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>