ipv4: fix ip_rt_update_pmtu()

commit 2c8cec5c10bc (Cache learned PMTU information in inetpeer) added
an extra inet_putpeer() call in ip_rt_update_pmtu().

This results in various problems, since we can free one inetpeer, while
it is still in use.

Ref: http://www.spinics.net/lists/netdev/msg159121.html

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