datapath: Don't directly access port array in query_port().

datapath: Don't directly access port array in query_port().

query_port() directly accesses the datapath port array, without
using any kind of RCU dereference.  It's OK, since it is holding
DP mutex but this adds an explicit check to make sparse happy.
It also simplifies the code path somewhat.

Found with sparse.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>