src.ctf.fs: remove goto error handling from fs.cpp

src.ctf.fs: remove goto error handling from fs.cpp

gotos prevent declaring variables in the middle of functions, which is
sometimes necessary in C++.  Remove gotos used for error handling,
replace them with returning directly.

Change-Id: I15b5d6aa4897e5326fc03e2626045c83258d8eca
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8318
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>