econet: fix locking

econet lacks proper locking. It holds econet_lock only when inserting or
deleting an entry in econet_sklist, not during lookups.

- convert econet_lock from rwlock to spinlock

- use econet_lock in ec_listening_socket() lookup

- use appropriate sock_hold() / sock_put() to avoid corruptions.

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