lib/vsprintf: refactor duplicate code to special_hex_number()

special_hex_number() is a helper to print a fixed size type in a hex
format with '0x' prefix, zero padding, and small letters.  In the module
we have already several copies of such code.  Consolidate them under
special_hex_number() helper.

There are couple of differences though.

It seems nobody cared about the output in case of CONFIG_KALLSYMS=n,
when printing symbol address, because the asked field width is not
enough to care last 2 characters in the string represantation of the
pointer.  Fixed here.

The %pNF specifier used to be allowed with a specific field width,
though there is neither any user of it nor mention the possibility in
the documentation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>