netlink: Use rhashtable walk interface in diag dump

This patch converts the diag dumping code to use the rhashtable
walk code instead of going through rhashtable by hand.  The lock
nl_table_lock is now only taken while we process the multicast
list as it's not needed for the rhashtable walk.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>