Merge branch 'mptcp-simplify-mptcp_accept'

Paolo Abeni says:

====================
mptcp: simplify mptcp_accept()

Currently we allocate the MPTCP master socket at accept time.

The above makes mptcp_accept() quite complex, and requires checks is several
places for NULL MPTCP master socket.

These series simplify the MPTCP accept implementation, moving the master socket
allocation at syn-ack time, so that we drop unneeded checks with the follow-up
patch.

v1 -> v2:
- rebased on top of e106062d02ae2e497e616bf728aafe34867c5ca7
====================

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