selftests/ftrace: Use printf for backslash included command

Since the built-in echo has different behavior in POSIX shell
(dash) and bash, kprobe_syntax_errors.tc can fail on dash which
interpret backslash escape automatically.

To fix this issue, we explicitly use printf "%s" (not interpret
backslash escapes) if the command string can include backslash.

Reported-by: Liu Yiding <yidingx.liu@intel.com>
Suggested-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>