librbd: fixed object map issues discovered via fsx

librbd: fixed object map issues discovered via fsx

The object map wasn't being properly refreshed after setting
the snapshot context on the parent image. Additionally fixed
a potential deadlock that could have occurred if no object
map update was required when trimming an image.

Fixes: #10706
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Reviewed-by: Josh Durgin <jdurgin@redhat.com>