[NETNS]: Lookup in FIB semantic hashes taking into account the namespace.

The namespace is not available in the fib_sync_down_addr, add it as a
parameter.

Looking up a device by the pointer to it is OK. Looking up using a
result from fib_trie/fib_hash table lookup is also safe. No need to
fix that at all.  So, just fix lookup by address and insertion to the
hash table path.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>