net: pmtu_expires fixes

commit 2c8cec5c (ipv4: Cache learned PMTU information in inetpeer)
added some racy peer->pmtu_expires accesses.

As its value can be changed by another cpu/thread, we should be more
careful, reading its value once.

Add peer_pmtu_expired() and peer_pmtu_cleaned() helpers

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