eCryptfs: Fix encrypted file name lookup regression

eCryptfs: Fix encrypted file name lookup regression

Fixes a regression caused by b0aeaa5c5188ab7515fe2e996b331ccad12e11e3

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>