fix sscanf %n match at end of input string

I was playing with some code that sometimes got a string where a %n
match should have been done where the input string ended, for example
like this:

  sscanf("abc123", "abc%d%n", &a, &n);  /* doesn't work */
  sscanf("abc123a", "abc%d%n", &a, &n); /* works */

However, the scanf function in the kernel doesn't convert the %n in that
case because it has already matched the complete input after %d and just
completely stops matching then. This patch fixes that.

[akpm@linux-foundation.org: cleanups]
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>