dtrace: no longer expose kuid_t in the userspace dtrace API

dtrace: no longer expose kuid_t in the userspace dtrace API

The public header installed as <linux/dtrace/stability.h> exposed
<linux/uidgid.h> to userspace as part of the dtrace_ppriv_t.dtpp_uid member.
This member (used for unprivileged tracing) is part of a facility that is not
yet ported, but using a kuid_t for this is clearly wrong, and as of kernel 4.0
won't compile when used in userspace either.

Fix by migrating to a uid_t and converting it to a kuid at the point of use.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Acked-by: Kris Van Hees <kris.van.hees@oracle.com>