mptcp: Do TCP fallback on early DSS checksum failure
RFC 8684 section 3.7 describes several opportunities for a MPTCP connection to "fall back" to regular TCP early in the connection process, before it has been confirmed that MPTCP options can be successfully propagated on all SYN, SYN/ACK, and data packets. If a peer acknowledges the first received data packet with a regular TCP header (no MPTCP options), fallback is allowed. If the recipient of that first data packet finds a MPTCP DSS checksum error, this provides an opportunity to fail gracefully with a TCP fallback rather than resetting the connection (as might happen if a checksum failure were detected later). This commit modifies the checksum failure code to attempt fallback on the initial subflow of a MPTCP connection, only if it's a failure in the first data mapping. In cases where the peer initiates the connection, requests checksums, is the first to send data, and the peer is sending incorrect checksums (see https://github.com/multipath-tcp/mptcp_net-next/issues/275 ), this allows the connection to proceed as TCP rather than reset. Fixes: dd8bcd17 ("mptcp: validate the data checksum") Acked-by:Paolo Abeni <pabeni@redhat.com> Signed-off-by: