Fix IPv6 OXMHeader Mask issue

When programming an OVS flow to set an ipv6_src/dest as
an action field (i.e., set_field:2001::1->ipv6_src), the
current implementation is not checking the Mask and is
including the Mask (even for a full ipv6 address) in the
OXM header. Because of this, switch is rejecting the flow
with the following error.

"decode error: OFPBAC_BAD_SET_MASK. OXM header 0:32768:26:1:32
includes mask but masked OXMs are not allowed here"