eCryptfs: ecryptfs_keyring_auth_tok_for_sig() bug fix

eCryptfs: ecryptfs_keyring_auth_tok_for_sig() bug fix

BugLink: http://bugs.launchpad.net/bugs/761134

commit 1821df040ac3cd6a57518739f345da6d50ea9d3f upstream.

The pointer '(*auth_tok_key)' is set to NULL in case request_key()
fails, in order to prevent its use by functions calling
ecryptfs_keyring_auth_tok_for_sig().

Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>