mptcp: fix duplicate subflow creation

Fullmesh endpoints could end-up unexpectedly generating duplicate
subflows - same local and remote addresses - when multiple incoming
ADD_ADDR are processed before the PM creates the subflow for the local
endpoints.

Address the issue explicitly checking for duplicates at subflow
creation time.

To avoid a quadratic computational complexity, track the unavailable
remote address ids in a temporary bitmap and initialize such bitmap
with the remote ids of all the existing subflows matching the local
address currently processed.

The above allows additionally replacing the existing code checking
for duplicate entry in the current set with a simple bit test
operation.

Fixes: 2843ff6f ("mptcp: remote addresses fullmesh")
Cc: stable@vger.kernel.org
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/435


Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>