tty: remove buffer special casing

Long long ago a 4K kmalloc allocated two pages so the tty layer used the
page allocator, except on some machines where the page size was huge. This was
removed from the core tty layer with the tty buffer re-implementation but not
from tty_audit or the n_tty ldisc.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>