net: factor out __inet_listen_sk() helper

The mptcp protocol maintains an additional socket just to easily
invoke a few stream operations on the first subflow. One of them
is inet_listen().

Factor out an helper operating directly on the (locked) struct sock,
to allow get rid of the above dependency in the next patch without
duplicating the existing code.

No functional changes intended.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>