mac_auth(). The size of a buffer
was miscomputed.

see http://bugzilla.kernel.org/show_bug.cgi?id=7245

Acked-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: Laurent Riffard <laurent.riffard@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>