[IA64] kprobe: make kreturn probe handler stack unwind correct

Restore regs->ccr_iip before kreturn probe handler runs. In this way, if
probe handler does unwind, unwind can correctly get the stack trace.

Fixes: http://sourceware.org/bugzilla/show_bug.cgi?id=5051

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>