backend: fix potential divide-by-zero

Signed-off-by: Jens Axboe <axboe@fb.com>
init: remove dead code

Signed-off-by: Jens Axboe <axboe@fb.com>
Error check fcntl() calls

Add a helper to mark an fd as non-blocking, so we can check
and complain in one place.

Signed-off-by: Jens Axboe <axboe@fb.com>
make_filename: another off-by-one buffer issue

Signed-off-by: Jens Axboe <axboe@fb.com>
num2str: ensure we never access postfix[] out-of-bounds

Signed-off-by: Jens Axboe <axboe@fb.com>