bridge: Don't complain if an expected cgroup file goes away
bridge: Don't complain if an expected cgroup file goes away

This can happen while walking the cgroup tree.

Fixes warnings like this:

error loading file: /sys/fs/cgroup/memory/system.slice/sys-kernel-debug.mount/memory.usage_in_bytes: Error reading file '/sys/fs/cgroup/memory/system.slice/sys-kernel-debug.mount/memory.usage_in_bytes': No such device

or

error loading file: /sys/fs/cgroup/memory/system.slice/sys-kernel-debug.mount/memory.memsw.usage_in_bytes: Failed to open file '/sys/fs/cgroup/memory/system.slice/sys-kernel-debug.mount/memory.memsw.usage_in_bytes': No such file or directory

Closes #2828
Reviewed-by: Marius Vollmer <bb11c6e942c5cc765df5f04da146b3198127a7a3@redhat.com>

deryni/cockpit,jscotka/cockpit,petervo/cockpit,stefwalter/cockpit,sgallagher/cockpit,FireDrunk/cockpit,sgallagher/cockpit,SotolitoLabs/cockpit,andreasn/cockpit,larsu/cockpit,petervo/cockpit,FireDrunk/cockpit,sgallagher/cockpit,matobet/cockpit,dperpeet/cockpit,larsu/cockpit,Scribery/cockpit,michalskrivanek/cockpit,matobet/cockpit,mvollmer/cockpit,andreasn/cockpit,moraleslazaro/cockpit,jscotka/cockpit,deryni/cockpit,petervo/cockpit,xhad/cockpit,Scribery/cockpit,larsu/cockpit,jscotka/cockpit,mareklibra/cockpit,Scribery/cockpit,haiyangd/cockpit_view,stefwalter/cockpit,deryni/cockpit,garrett/cockpit,harishanand95/cockpit,harishanand95/cockpit,michalskrivanek/cockpit,deryni/cockpit,sgallagher/cockpit,SotolitoLabs/cockpit,sgallagher/cockpit,mvollmer/cockpit,garrett/cockpit,andreasn/cockpit,dperpeet/cockpit,harishanand95/cockpit,SotolitoLabs/cockpit,andreasn/cockpit,andreasn/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,vanloswang/cockpit,dperpeet/cockpit,mareklibra/cockpit,garrett/cockpit,haiyangd/cockpit_view,cockpituous/cockpit,FireDrunk/cockpit,vanloswang/cockpit,xhad/cockpit,arilivigni/cockpit,matobet/cockpit,xhad/cockpit,martinpitt/cockpit,larskarlitski/cockpit,michalskrivanek/cockpit,arilivigni/cockpit,vanloswang/cockpit,vanloswang/cockpit,cockpituous/cockpit,zigitax/cockpit,harishanand95/cockpit,michalskrivanek/cockpit,garrett/cockpit,larsu/cockpit,jscotka/cockpit,Scribery/cockpit,harishanand95/cockpit,Scribery/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,xhad/cockpit,xhad/cockpit,FireDrunk/cockpit,arilivigni/cockpit,harishanand95/cockpit,vanloswang/cockpit,Scribery/cockpit,haiyangd/cockpit_view,mareklibra/cockpit,dperpeet/cockpit,vanloswang/cockpit,xhad/cockpit,petervo/cockpit,deryni/cockpit,dperpeet/cockpit,zigitax/cockpit,zigitax/cockpit,michalskrivanek/cockpit,andreasn/cockpit,mvollmer/cockpit,cockpituous/cockpit,deryni/cockpit,mareklibra/cockpit,michalskrivanek/cockpit,arilivigni/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,larsu/cockpit,moraleslazaro/cockpit,larskarlitski/cockpit,SotolitoLabs/cockpit,andreasn/cockpit,petervo/cockpit,SotolitoLabs/cockpit,stefwalter/cockpit,cockpit-project/cockpit,jscotka/cockpit,cockpit-project/cockpit,martinpitt/cockpit,sgallagher/cockpit,cockpit-project/cockpit,garrett/cockpit,larskarlitski/cockpit,stefwalter/cockpit,xhad/cockpit,larskarlitski/cockpit,petervo/cockpit,mvollmer/cockpit,Scribery/cockpit,martinpitt/cockpit,deryni/cockpit,larskarlitski/cockpit,mareklibra/cockpit,SotolitoLabs/cockpit,cockpituous/cockpit,mareklibra/cockpit,cockpituous/cockpit,stefwalter/cockpit,haiyangd/cockpit_view,larskarlitski/cockpit,martinpitt/cockpit,dperpeet/cockpit,mvollmer/cockpit,zigitax/cockpit,mareklibra/cockpit,petervo/cockpit,vanloswang/cockpit,sgallagher/cockpit,jscotka/cockpit,harishanand95/cockpit,matobet/cockpit,moraleslazaro/cockpit,larskarlitski/cockpit,arilivigni/cockpit,dperpeet/cockpit,arilivigni/cockpit,stefwalter/cockpit,FireDrunk/cockpit,zigitax/cockpit,haiyangd/cockpit_view,larsu/cockpit,SotolitoLabs/cockpit,matobet/cockpit,martinpitt/cockpit,arilivigni/cockpit,matobet/cockpit,stefwalter/cockpit,michalskrivanek/cockpit