mptcp: fix checksum byte order
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2100072 Tested: LNST and self-tests
The issue is addressed by the first patch, while the 2nd one improves interoperability with bugged hosts allowing TCP fallback in such scenario.
Signed-off-by: Paolo Abeni pabeni@redhat.com