UBUNTU: zfs: Fix user namespaces uid/gid mapping

UBUNTU: zfs: Fix user namespaces uid/gid mapping

BugLink: http://bugs.launchpad.net/bugs/1567558
Cherry-picked from 874bd959f4f15b3d4b007160ee7ad3f4111dd341 ('Fix user
namespaces uid/gid mapping')
https://github.com/zfsonlinux/zfs.git

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>
Signed-off-by: Massimo Maggi <me@massimo-maggi.eu>
Closes #4177
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>