sctp: sctp_transport_lookup_process should rcu_read_unlock when transport is null

Prior to this patch, sctp_transport_lookup_process didn't rcu_read_unlock
when it failed to find a transport by sctp_addrs_lookup_transport.

This patch is to fix it by moving up rcu_read_unlock right before checking
transport and also to remove the out path.

Fixes: 1cceda78 ("sctp: fix the issue sctp_diag uses lock_sock in rcu_read_lock")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>