mptcp: add annotations around msk->subflow accesses

mptcp: add annotations around msk->subflow accesses

BugLink: https://bugs.launchpad.net/bugs/2029808
[ Upstream commit 5b825727d0871b23e8867f6371183e61628b4a26 ]

The MPTCP can access the first subflow socket in a few spots
outside the socket lock scope. That is actually safe, as MPTCP
will delete the socket itself only after the msk sock close().

Still the such accesses causes a few KCSAN splats, as reported
by Christoph. Silence the harmless warning adding a few annotation
around the relevant accesses.

Fixes: 71ba088ce0aa ("mptcp: cleanup accept and poll")
Reported-by: Christoph Paasch <cpaasch@apple.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/402
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>