Fix race condition in smart proxy.

Ignore EEXIST error if directory is created by another thread after existence check.

https://bugzilla.redhat.com/show_bug.cgi?id=1148971
https://bugzilla.redhat.com/show_bug.cgi?id=1153784
Merge pull request #1113 from roliveri/smart_proxy_race_condition

Fix race condition in smart proxy.