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>
Merge pull request #2980 from ceph/wip-10123

librbd: protect list_children from invalid child pool IoCtxs

Reviewed-by: Josh Durgin <jdurgin@redhat.com>