This is a followup to 38a572011d ("qa: rbd/map-snapshot-io.sh: don't
chown sysfs files") for the rest of the workunits:
- /sys/bus/rbd/add_single_major is used if present, so chown'ing
/sys/bus/rbd/add doesn't help. Use sudo on rbd map instead.
- Don't modprobe or udevadm settle - rbd CLI tool takes care of that.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>