Merge branch 'mptcp-some-fallback-fixes'

Paolo Abeni says:

====================
mptcp: some fallback fixes

pktdrill pointed-out we currently don't handle properly some
fallback scenario for MP_JOIN subflows

The first patch addresses such issue.

Patch 2/2 fixes a related pre-existing issue that is more
evident after 1/2: we could keep using for MPTCP signaling
closed subflows.
====================

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