Commit 96f05a7956 ("rbd: delay determination of default pool name")
broke "rbd perf image iostat" and "rbd perf image iotop" GLOBAL_POOL_KEY
support (the ability to blend all rbd pools together into a single
view).
Fixes: https://tracker.ceph.com/issues/56561
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>