src.ctf.fs: fs.cpp: declare variables on first use

src.ctf.fs: fs.cpp: declare variables on first use

Move variable declarations to be at or closer to their first use.

Change-Id: Iec7b1789a9cd007938b992292ad48a09d23f79e5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8403
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12346
Tested-by: jenkins <jenkins@lttng.org>