This does all the RCU magic inside of it. The caller must call put_cred() on
the credentials so obtained when they're finished with.
+ [*] Note: The result of __task_cred() should not be passed directly to
+ get_cred() as this may race with commit_cred().
+
There are a couple of convenience functions to access bits of another task's
credentials, hiding the RCU magic from the caller: