selftests/ftrace: Make uprobe test more robust against binary name

Make add_remove_uprobe test case more robust against various real
binary name.
Current add_remove_uprobe.tc test expects the real binary of /bin/sh
is '*/bin/*sh', but it does not work on busybox environment.
Instead of using fixed pattern, use readlink to identify real binary
name.

Link: https://lore.kernel.org/r/173625187633.1383744.2840679071525852811.stgit@devnote2


Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>