tomoyo: use vsnprintf() properly

tomoyo: use vsnprintf() properly

Idiomatic way to find how much space sprintf output would take is
	len = snprintf(NULL, 0, ...) + 1;
Once upon a time there'd been libc implementations that blew chunks
on that and somebody had come up with the following "cute" trick:
	len = snprintf((char *) &len, 1, ...) + 1;
for doing the same.  However, that's unidiomatic, harder to follow
*and* any such libc implementation would violate both C99 and POSIX
(since 2001).
	IOW, this kludge is best buried along with such libc implementations,
nevermind getting cargo-culted into newer code.  Our vsnprintf() does not
suffer that braindamage, TYVM.

Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>