ofproto-dpif: Unset DPIF_FP_MODIFY flag when creating a new flo

Remove the DPIF_FP_MODIFY flag when creating a new flow. When flows arrive in
a batch, userspace may push down multiple unique flow definitions that
overlap when wildcards are applied. Kernels support flow wildcarding
will reject these flow as duplicates (EEXIST), which will be logged
at a lower logging level.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>