ospf6d: Increment sequence number correctly in Authentication Trailer

ospf6d: Increment sequence number correctly in Authentication Trailer

According to RFC 7166, the sequence number should be treated as an
unsigned 64-bit value, although it is stored as two 32-bit values.

When incrementing it, the code caused the lower-order 32-bit value
to skip from 0xFFFFFFFE to 0. As a side effect, an error was never
produced if the full 64-bit sequence number wrapped.

Fixes: #13805
Signed-off-by: David Ward <david.ward@ll.mit.edu>