Merge branch 'mptcp-deadlock'

Paolo Abeni says:

====================
mptcp: mptcp: fix deadlock in mptcp{,6}_release

syzkaller has reported a few deadlock triggered by
mptcp{,6}_release.

These patches address the issue in the easy way - blocking
the relevant, multicast related, sockopt options on MPTCP
sockets.

Note that later on net-next we are going to revert patch 1/2,
as a part of a larger MPTCP sockopt implementation refactor
====================

Signed-off-by: David S. Miller <davem@davemloft.net>