ofproto-dpif-upcall: Fix use of cleared stack memory.

Commit cc377352d (ofproto: Reorganize in preparation for direct
dpdk upcalls.) introduced the bug that uses variable defined on
the stack inside while loop for reading dpif upcalls and keeps
reference to attributes of the variable within the same function
after the stack is cleared.  This bug can cause ovs abort.

This commit fixes the above issue by defining an array of the
variable on the function stack.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>