bridge: fix possible overflow in get_fdb_entries (CVE-2006-5751)

Make sure to properly clamp maxnum to avoid overflow (CVE-2006-5751).

Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Acked-by: Stephen Hemminger <shemminger@osdl.org>
Acked-by: David Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>