debug/io-stats: Handle open failures gracefully

Change-Id: I85b22db5cc456b3e8c9f26c8254f08a796fc2b28
BUG: 903336
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4418
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>