Check `so_state' in rtm_senddesync() and return if SS_ISCONNECTED or

Check `so_state' in rtm_senddesync() and return if SS_ISCONNECTED or
SS_CANTRCVMORE bits are set.

The first check required to prevent timeout_add(9) reschedule
`rop_timeout', otherwise timeout_del_barrier(9) can't help us.

The second check is for the case when shutdown(2) with SHUT_RD argument
occurred on this socket and we should not receive anything include
RTM_DESYNC packets.

ok claudio@