decnet: Use TCP nagle macro instead of literal number in decnet

Use existing TCP nagle macro TCP_NAGLE_OFF and TCP_NAGLE_CORK instead
of the literal number 1 and 2 in the current decnet codes.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>