net: move rollback_registered_many()

commit bcfe2f1a upstream.

Move rollback_registered_many() and add a temporary
forward declaration to make merging the code into
unregister_netdevice_many() easier to review.

No functional changes.

Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>