AUDIT: expand audit tmp buffer as needed

Introduce audit_expand and make the audit_buffer use a dynamic buffer
which can be resized.  When audit buffer is moved to skb it will not
be fragmented across skb's, so we can eliminate the sklist in the
audit_buffer.  During audit_log_move, we simply copy the full buffer
into a single skb, and then audit_log_drain sends it on.

Signed-off-by: Chris Wright <chrisw@osdl.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>