datapath: Use multicast groups allocated for upcalls.

datapath: Use multicast groups allocated for upcalls.

We allocate a number of multicast groups and stripe upcalls across
them using a hash function.  However, instead of using the ID of
the selected group for the upcall multicast we were directly using
the output of the hash function.  In the best case this leads to
intermittent failures when we choose an invalid group ID (such as
0) or in the worse case could lead to access of unallocated memory.
This fixes that by looking up the group we have been allocated.

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