[NETFILTER]: nf_conntrack_{tcp,sctp}: shrink state table

The TCP and SCTP conntrack state transition tables only holds
small numbers, but gcc uses 4 byte per entry for the enum. Switching
to an u8 reduces the size from 480 to 120 bytes for TCP and from
576 to 144 bytes for SCTP.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>