ipv4: fix rcu lock imbalance in fib_select_default()

Commit 0c838ff1 (ipv4: Consolidate all default route selection
implementations.) forgot to remove one rcu_read_unlock() from
fib_select_default().

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>