Remove passing by reference to allow for proper GC

The garbage collector in PHP 5.3.10 does not properly release the file
handle when calling fclose() due to the fact that it is passed by
reference.
This has the side-effect of preventing file locks to be released as well
when the files_locking app is enabled.

This fix removes the useless passing by reference and now the file
handle and file lock are freed properly.
Merge pull request #11050 from owncloud/enc-flockfix

Remove passing by reference to allow for proper GC