ipv6: fib: Dump tables during registration to FIB chain

Dump all the FIB tables in each net namespace upon registration to the
FIB notification chain so that the callee will have a complete view of
the tables.

The integrity of the dump is ensured by a per-table sequence counter
that is incremented (under write lock) whenever a route is added or
deleted from the table.

All the sequence counters are read (under each table's read lock) and
summed, prior and after the dump. In case the counters differ, then the
dump is either restarted or the registration fails.

While it's possible for a table to be modified after its counter has
been read, this isn't really a problem. In case it happened before it
was read the second time, then the comparison at the end will fail. If
it happened afterwards, then we're guaranteed to be notified about the
change, as the notification block is registered prior to the second
read.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>