ftrace/selftests: Change non-unique test symbol to init_once

The name_show symbol is not the best choise, because it often
appears only three times on s390 in kallsyms, with two of them
having module annotations [mdev] and [i2c_core].

Switch to init_once which has many more duplicate instances
all in vmlinux, making it a more reliable non-unique symbol
for testing kprobe registration failure.

Link: https://lore.kernel.org/all/67ac21cd214ec9371276df9ba790e9832cf328d4.1787670777.git.agordeev@linux.ibm.com/

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>