ofproto-dpif-upcall: Fix out-of-scope use of stack memory.

Commit cc377352d (ofproto: Reorganize in preparation for direct
dpdk upcalls.) introduced the bug that keeps reference to 'struct
flow' defined on the stack inside while loop when running out of
the scope.  This causes strange bug like wrong mask extraction
when the part of memory is corrupted, and could lead to even
more serious bug/crash.

This commit fixes the above issue by defining an array of the
'struct flow's on the function stack.

Found by running ovs on RHEL7.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>