vsprintf: neaten %pK kptr_restrict, save a bit of code space

If kptr restrictions are on, just set the passed pointer to NULL.

$ size lib/vsprintf.o.*
   text	   data	    bss	    dec	    hex	filename
   8247	      4	      2	   8253	   203d	lib/vsprintf.o.new
   8282	      4	      2	   8288	   2060	lib/vsprintf.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Dan Rosenberg <drosenberg@vsecurity.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>