Fetch the sequence number the way we fetch the acknowledgment number.

Fetch the sequence number the way we fetch the acknowledgment number.

Fetch 24 bits if the X bit isn't set, 48 bits if it is, using the
appropriate EXTRACT_ macros.

We do this with "struct dccp_hdr" being a header structure with a 24-bit
sequence number and "struct dccp_hdr_ext" being a header structure with
a 48-bit sequence number.