As described in torvalds/linux@5f3a4a2 the &init_user_ns, and
not the current user_ns, should be passed to posix_acl_from_xattr()
and posix_acl_to_xattr(). Conveniently the init_user_ns is
available through the init credential (kcred).
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4177