MFC r358621:

When using automatically generated flow labels and using TCP SYN
cookies, use the same flow label for the segments sent during the
handshake and after the handshake.
This fixes a bug by making sure that sc_flowlabel is always stored in
network byte order.

Reviewed by: bz
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D23957