Prior to this commit, gocryptfs's reverse mode did not report correct
directory entry sizes for symbolic links, where the dentry size needs to
be the same as the length of a string containing the target path.
This commit corrects this issue and adds a test case to verify the
correctness of the implementation.
This issue was discovered during the use of a strict file copying program
on a reverse-mounted gocryptfs file system.
d48ccb3dda