librbd: protect list_children from invalid child pool IoCtxs

While listing child images, don't ignore error codes returned
from librados when creating an IoCtx. This will prevent seg
faults from occurring when an invalid IoCtx is used.

Fixes: #10123
Backport: giant, firefly, dumpling
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 0d350b6817d7905908a4e432cd359ca1d36bab50)
Merge pull request #3064 from ceph/wip-10123-dumpling

librbd: protect list_children from invalid child pool IoCtxs

Reviewed-by: Sage Weil <sage@redhat.com>