ktest: cleanup get_grub_index

Cleanup get_grub_index().

Link: http://lkml.kernel.org/r/20190509213647.6276-3-msys.mizuma@gmail.com



Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>