eCryptfs: Fix encrypted file name lookup regression

Fixes a regression caused by 21edad32205e97dc7ccb81a85234c77e760364c8

When file name encryption was enabled, ecryptfs_lookup() failed to use
the encrypted and encoded version of the upper, plaintext, file name
when performing a lookup in the lower file system. This made it
impossible to lookup existing encrypted file names and any newly created
files would have plaintext file names in the lower file system.

https://bugs.launchpad.net/ecryptfs/+bug/623087

Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>