tipc: Expand link sequence gap field to 13 bits

This patch increases the "sequence gap" field of the LINK_PROTOCOL
message header from 8 bits to 13 bits (utilizing 5 previously
unused 0 bits).  This ensures that the field is big enough to
indicate the loss of up to 8191 consecutive messages on the link,
thereby accommodating the current worst-case scenario of 4000
lost messages.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>