mptcp: avoid unneeded address copy

[ Upstream commit 2bb9a37f ]

In the syn_recv fallback path, the msk is unused. We can skip
setting the socket address.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 7e8b88ec ("mptcp: consolidate passive msk socket initialization")
Signed-off-by: Sasha Levin <sashal@kernel.org>