x86: expand irq-off region in text_poke()

Expand irq-off region to cover fixmap using code and cache synchronizing.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
LKML-Reference: <49B54688.8090403@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>