mptcp: diag: switch to context structure

[ Upstream commit 6b9ea5c8 ]

Raw access to cb->arg[] is deprecated, use a context structure.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 871019b2 ("net: set SOCK_RCU_FREE before inserting socket into hashtable")
Signed-off-by: Sasha Levin <sashal@kernel.org>