tty: Merge tty_buffer_find() into tty_buffer_alloc()

tty_buffer_find() implements a simple free list lookaside cache.
Merge this functionality into tty_buffer_alloc() to reflect the
more traditional alloc/free symmetry.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>