net: fix iteration for sctp transport seq_files

The sctp transport seq_file iterators take a reference to the transport
in the ->start and ->next functions and releases the reference in the
->show function.  The preferred handling for such resources is to
release them in the subsequent ->next or ->stop function call.

Since Commit 1f4aace6 ("fs/seq_file.c: simplify seq_file iteration
code and interface") there is no guarantee that ->show will be called
after ->next, so this function can now leak references.

So move the sctp_transport_put() call to ->next and ->stop.

Fixes: 1f4aace6

 ("fs/seq_file.c: simplify seq_file iteration code and interface")
Reported-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>