Linux: Get rid of !STRUCT_KEY_UID_IS_KUID_T case

Linux: Get rid of !STRUCT_KEY_UID_IS_KUID_T case

On the few kernel versions before struct key.uid was converted to
kuid_t (v3.7-rc1~147^2~76), it was not possible to enable both
CONFIG_KEYS and CONFIG_UIDGID_STRICT_TYPE_CHECKS, so this case was
impossible.  That’s good, because it also had a typo in its
implementation (and was confusing to deal with correctly).

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/10443
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit d0a13fe678412464452afae9379d63fa48d41d83)

Change-Id: I081115d13b6deb3b10e6da442bed7a7bb9347296
Reviewed-on: http://gerrit.openafs.org/10458
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>