[IA64] Fix the order of atomic operations in restore_previous_kprobes on ia64

Fix the order of atomic operations to prevent overwriting prev_kprobe[0].
To pop values from stack, we must decrement stack index right AFTER
reading values.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>