lib/test_printf.c: check for out-of-bound writes

lib/test_printf.c: check for out-of-bound writes

Add a few padding bytes on either side of the test buffer, and check
that these (and the part of the buffer not used) are untouched by
vsnprintf.

Signed-off-by: Rasmus Villemoes <[email protected]>
Acked-by: Kees Cook <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Maurizio Lombardi <[email protected]>
Cc: Tejun Heo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>