net: bridge: remove redundant check of f->dst

In br_fill_forward_path(), f->dst is checked not to be NULL, then
immediately read using READ_ONCE and checked again. The first check is
useless, so this patch aims to remove the redundant check of f->dst.

Signed-off-by: linke li <lilinke99@qq.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>