mptcp: export mptcp_subflow_early_fallback()

This helper will be used outside protocol.h in the following commit.

While at it, also add a 'pr_fallback()' debug print, to help identifying
fallbacks.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240909-net-next-mptcp-fallback-x-mpc-v1-1-da7ebb4cd2a3@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>