lib/vsprintf.c: warn about too large precisions and field widths

The field width is overloaded to pass some extra information for some %p
extensions (e.g.  #bits for %pb).  But we might silently truncate the
passed value when we stash it in struct printf_spec (see e.g.
"lib/vsprintf.c: expand field_width to 24 bits").  Hopefully 23 value
bits should now be enough for everybody, but if not, let's make some
noise.

Do the same for the precision.  In both cases, clamping seems more
sensible than truncating.  While, according to POSIX, "A negative
precision is taken as if the precision were omitted.", the kernel's
printf has always treated that case as if the precision was 0, so we use
that as lower bound.  For the field width, the smallest representable
value is actually -(1<<23), but a negative field width means 'set the
LEFT flag and use the absolute value', so we want the absolute value to
fit.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Joe Perches <joe@perches.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Maurizio Lombardi <mlombard@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>