x86: fix text_poke()

kernel_text_address returns true even for modules which is not wanted
in text_poke. Use core_kernel_text instead.

This is a regression introduced in e587cadd
which caused occasionaly crashes after suspend/resume.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
CC: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: Andi Kleen <andi@firstfloor.org>
CC: pageexec@freemail.hu
CC: H. Peter Anvin <hpa@zytor.com>
CC: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>