lib/vsprintf.c: fix vscnprintf() if @size is == 0

vscnprintf() should return 0 if @size is == 0.  Update the comment for it,
as @size is unsigned.

This change based on the code of commit
b903c0b8

 ("lib: fix scnprintf() if @size
is == 0") moves the real fix into vscnprinf() from scnprintf() and makes
scnprintf() call vscnprintf(), thus avoid code duplication.

Signed-off-by: Anton Arapov <aarapov@redhat.com>
Acked-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>