datapath: fix a bug in SF_FLOW_KEY_PUT macro

datapath: fix a bug in SF_FLOW_KEY_PUT macro

This bug will cause mask values to corrupt the flow key value. So far
the bug has not showed up because we don't write mask value when
there is no mask Netlink attributes.  However, it needs to be fixed for
the next and future commits where we will start to set default
values for key and mask for missing Netlink attributes.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>