mptcp: properly annotate nested lock

MPTCP closes the subflows while holding the msk-level lock.
While acquiring the subflow socket lock we need to use the
correct nested annotation, or we can hit a lockdep splat
at runtime.

Reported-and-tested-by: Geliang Tang <geliangtang@gmail.com>
Fixes: e16163b6 ("mptcp: refactor shutdown and close")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>