ospf6d: Fix double increment of Sequence Number

ospf6d: Fix double increment of Sequence Number

When OSPF6 is creating the header for the ROUTER LSA type
if the packet being sent has interface information to add
to the data, the Sequence Number is at least double incremented.

This change moves the header creation to outside the loop over
all interfaces in the area.  Additionally the header is created
at the bottom of the function now.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>