lib/vsprintf.c: fix interaction of kasprintf() and vsnprintf() when using %pV

Otherwise, the warning at the top of vsnprintf() gets triggered by
kvasprintf()'s first invocation (with NULL buffer and zero size) of
vsnprintf().

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>