decnet: fix shutdown parameter checking

The allowed value of "how" is SHUT_RD/SHUT_WR/SHUT_RDWR (0/1/2),
rather than SHUTDOWN_MASK (3).

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>