Linux 4.8 compat: new s_user_ns member of struct super_block

Kernel 4.8 paved the way to enabling mounting a file system inside a
non-init user namespace. To facilitate this a s_user_ns member was
added holding the userns in which the filesystem's instance was
mounted. This enables doing the uid/gid translation relative to
this particular username space and not the default init_user_ns.

Signed-off-by: Nikolay Borisov <n.borisov.lkml@gmail.com>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4928