mptcp: track accurately the incoming MPC suboption type

Currently in the receive path we don't need to discriminate
between MPC SYN, MPC SYN-ACK and MPC ACK, but soon the fastopen
code will need that info to properly track the fully established
status.

Track the exact MPC suboption type into the receive opt bitmap.
No functional change intended.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>