ecryptfs: Return an error code only as a constant in ecryptfs_add_global_auth_tok()

* Return an error code without storing it in an intermediate variable.

* Delete the jump target "out" and the local variable "rc"
  which became unnecessary with this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>