BugLink: http://bugs.launchpad.net/bugs/1570906

Two kernfs-based filesystems are currently mountable from
non-init user namespaces, sysfs and cgroupfs. Both of these have
use cases which end up attempting to mount the same superblock
from different user namespaces, but sget() no longer allows this
so these mount attempts will fail.

Update kernfs_test_super() to return false whenever s_user_ns is
not the current user namespace, resulting in a new super block
being allocated for these mounts. kernfs, sysfs, and cgroupfs are
all equipped to handle this situation.
Tested-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Leann Ogasawara <leann.ogasawara@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>