mptcp: more accurate NL event generation

Currently the mptcp code generate a "new listener" event even
if the actual listen() syscall fails. Address the issue moving
the event generation call under the successful branch.

Cc: stable@vger.kernel.org
Fixes: f8c9dfbd

 ("mptcp: add pm listener events")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230725-send-net-20230725-v1-2-6f60fe7137a9@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>