tipc: rename fields in struct tipc_link

We rename some fields in struct tipc_link, in order to give them more
descriptive names:

next_in_no -> rcv_nxt
next_out_no-> snd_nxt
fsm_msg_cnt-> silent_intv_cnt
cont_intv  -> keepalive_intv
last_retransmitted -> last_retransm

There are no functional changes in this commit.

Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>