eCryptfs: added support for the encrypted key type

The function ecryptfs_keyring_auth_tok_for_sig() has been modified in order
to search keys of both 'user' and 'encrypted' types.

Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Acked-by: Gianluca Ramunno <ramunno@polito.it>
Acked-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>