Fix: src.ctf.fs: increment packet offset before init of index entry

Issue
=====
The `current_packet_offset_bytes` variable is incremented before the
initialization of the current `index_entry` by `init_index_entry()`.
This lead to each entry having the offset of the next entry and the last
entry having a erroneous offset.

Solution
========
Increment the `current_packet_offset_bytes` variable after the
initialization of each entry.

Drawbacks
=========
None.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I7592b2d70f402aa413de97a687bfe4fe1bf1b825
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1409
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>