openvswitch: Fix output of SCTP mask.

The 'output' argument of the ovs_nla_put_flow() is the one from which
the bits are written to the netlink attributes.  For SCTP we
accidentally used the bits from the 'swkey' instead.  This caused the
mask attributes to include the bits from the actual flow key instead
of the mask.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>