iterator: save original next callback in iterator struct

iterator: save original next callback in iterator struct

When we restore the iterator's next method (after we overrode it with
post_auto_seek_next), we inspect the upstream component class to find
out what was the original method.  It seems simpler to save and restore
the function pointer.  This is what this patch does.

No user-visible changes intended.

Change-Id: If5eb7df692d2aa745f7f19e22c8046afb5b8d1d2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1485
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>