tipc: fix message importance range check

Commit 3b4f302d ("tipc: eliminate
redundant locking") introduced a bug by removing the sanity check
for message importance, allowing programs to assign any value to
the msg_user field. This will mess up the packet reception logic
and may cause random link resets.

Signed-off-by: Erik Hugne <erik.hugne@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>