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).

Change-Id: I4ecd164ed3604558ed4419bf6f9d531bd5d1a9ff
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>