mptcp: avoid unneeded __mptcp_nmpc_socket() usage

In a few spots, the mptcp code invokes the __mptcp_nmpc_socket() helper
multiple times under the same socket lock scope. Additionally, in such
places, the socket status ensures that there is no MP capable handshake
running.

Under the above condition we can replace the later __mptcp_nmpc_socket()
helper invocation with direct access to the msk->subflow pointer and
better document such access is not supposed to fail with WARN().

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>