ipvs: Restrict sync message to 255 connections

The nr_conns variable in the sync message header is only eight bits wide
and will overflow on interfaces with a large MTU. As a result the backup
won't parse all connections contained in the sync buffer. On regular
ethernet with an MTU of 1500 this isn't a problem, because we can't
overflow the value, but consider jumbo frames being used on a cross-over
connection between both directors.

We now restrict the size of the sync buffer, so that we never put more
than 255 connections into a single sync buffer.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Signed-off-by: Simon Horman <horms@verge.net.au>