Linux: Fix build with CONFIG_UIDGID_STRICT_TYPE_CHECKS (user namespaces)

With CONFIG_UIDGID_STRICT_TYPE_CHECKS (a dependency of user namespace
support, CONFIG_USER_NS) turned on, uid_t and kuid_t are different
types, as are gid_t and kgid_t, and we need to use namespace-dependent
functions to convert between them.

We can’t use init_user_ns as the namespace because it’s GPL-only, so
instead we grab the current user_ns at module load time.

This is required to support kernels with user namespace support.  We
don’t yet have full support for independent AFS use by different users
in a multiuser container; that will need to wait for future work.

Change-Id: Icc03f9098dd25b483d406db5167264ba960cdcb7
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/10386
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>