shared: create files even if the SELinux policy has no context for them

The SELinux policy defines no context for some files. E.g.:
  $ matchpathcon /run/lock/subsys /dev/mqueue
  /run/lock/subsys        <<none>>
  /dev/mqueue     <<none>>

We still need to be able to create them.
In this case selabel_lookup_raw() returns ENOENT. We should then skip
setfscreatecon(), but still return success.
It was broken since c34255bdb2 ("label: unify code to make directories,
symlinks").