LU-9003 mdc: support posix_acl_valid with user namespace

LU-9003 mdc: support posix_acl_valid with user namespace

Starting with Linux 4.8, posix_acl_valid takes an additional struct
user_namespace argument. Use init_user_ns since no other namespace is
available at this point (based on Linux commit
0d4d717f25834134bb6f43284f84c8ccee5bbf2a).

Change-Id: I656cb32f632c7ee00bdc36b6f6d4b442fcb03fcc
Signed-off-by: Michael Kuhn <michael.kuhn@informatik.uni-hamburg.de>
Reviewed-on: https://review.whamcloud.com/25307
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>