src.ctf.fs: compute stream range using entire file info group

Given that index entries are sorted within their file info structure and
that file info structures are sorted by time within the file info group,
we can simply compute the range of the stream by using the absolute
first index entry and the absolute last index entry of the entire file
info group.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I5b13451d72a4b1aeb9f31140ebfb025e2d712d7e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1128
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>