Prevent pointer to an out-of-scope local variable

show_str could be a pointer to a local variable in stack
which is out-of-scope by the time
return (snprintf(buf, buflen, "%s\n", show_str));
is called.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8924
Closes #8940