[NETFILTER]: attribute count is an attribute of message type, not subsytem

Prior to this patch, every nfnetlink subsystem had to specify it's
attribute count.  However, in reality the attribute count depends on
the message type within the subsystem, not the subsystem itself.  This
patch moves 'attr_count' from 'struct nfnetlink_subsys' into
nfnl_callback to fix this.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>